photography workshop

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Broad introduction to the field of photography, covering topics including human color perception, color theory, lighting, composition, how cameras work, optics, etc.Released under Creative Commons Attribution- Share Alike 3.0 United States License, this is an original work with illustrations sourced from the Wikimedia Foundation, and the author's own photographs.

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  • Photography WorkshopKen Stewart, creativexposure LLC

  • This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United States License. To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/3.0/us/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.

    Many of the images and diagrams used in this work are taken from the Wikimedia Commons (http://commons.wikimedia.org/wiki/Main_Page), and are reproduced under the terms of the GNU Free Documentation License, Version 1.2. Others as noted are used with the permission of their authors under the terms of the GNU Free Documentation License, Version 1.2.

    All remaining images are the authors own work and are licensed under the Creative Commons Attribution-Share Alike 3.0 United States License.

  • Modules Art & The Nature of Light Anatomy of The Camera Aperture, Time & Sensitivity Automation - Help or Hindrance? Focus on Lenses Lighting Putting It All Together Production - Before And After The Click

  • Art & The Nature of Light

  • photography (f-t!g'r-f")from Greek photos (!!"!#), light, and graphos ($%&!#), writing - n.

    1. The art or process of producing images of objects on photosensitive surfaces.

    2. The art, practice, or occupation of taking and printing photographs.

    3. A body of photographs.

  • Light

    Light is essential to vision, photography and all the visual arts

  • The Photographers View Light is what makes images possible Lights artistic properties include its color value (or hue),

    color purity (grayness or saturation), intensity (brightness), direction, spread (narrow beam, omnidirectional etc) diffuseness (hard or soft), duration (continuous, or in bursts), size/distance of the source relative to the subject

    Many of these properties are combined when we talk about different kinds of light in everyday usage - bright sunlight, cloudy daylight, open shade, fluorescent light, incandescent light

    For example, midday sunlight is white in color, very bright, comes from almost overhead, hard yet omnidirectional, continuous, etc

  • Hard or Soft Light?

    What kind of light are we seeing here?

    Hard or soft?

    Bright or dim?

    Directional?

    What color of light?

  • Hard or Soft Light?

    What kind of light are we seeing here?

    Hard or soft?

    Bright or dim?

    Directional?

    What color of light?

  • High or Low Key?

  • High or Low Key?

  • A Physicists View of Light Light is a form of electromagnetic radiation, comprised of photons,

    which have both wavelike and particle-like properties The four basic properties of light are reflection, refraction, diffraction,

    and interference Light rays travel in straight lines, but they be transmitted through

    different substances (or mediums), they can be bent (or refracted) when they travel from one medium to another, they can be reflected, absorbed or when they strike a surface, and they can be scattered (or diffracted) by obstacles and certain surfaces.

    When light of different colors (wavelengths) is refracted by different amounts (or dispersed) within a material, that material can be used to make a prism that splits white light into its components

    When light of one color hits some materials, it can cause the material to emit light of a different color - this is called fluorescence.

  • More On The Physics of Light In the vacuum of space, light travels at a maximum velocity (c) of

    299,792,458 meters per second, or about 186,000 miles per second. When light travels in anything other than a vacuum, like in air or glass, its speed will always be lower than the maximum

    Lights speed can increase (but never above c) or decrease when it travels from one medium to another. This change of the lights phase velocity is what causes refraction

    The primary physical properties of light are wavelength or frequency, intensity and polarization

    We perceive light of different wavelengths as having different colors, and its intensity as brightness. Many humans can learn to directly perceive polarization of light, and many animals can. Humans can see light with wavelengths in the range 400-700nm; birds and insects can often see light with shorter wavelengths into the ultra-violet, and some animals, eg pit vipers, can sense light with longer wavelengths, into the infra-red

  • The Electromagnetic Spectrum

  • White Light White is the color humans perceive when all

    three types of cone cells in the eye are stimulated in almost equal amounts, and with high brightness

    White light can be generated in many ways. The Sun, fire, and electric incandescence are thermal light sources. Other light sources such as fluorescent lamps and light-emitting diodes produce light by spontaneous emission

  • White Light & Color Temperature Thermal light sources give off a broad spectrum of frequencies

    (white light) characteristic of black-body radiation; Light from the Sun comes from its 6000K/10000F surface, the

    chromosphere, Incandescent light comes from the 2500K tungsten filament in

    a light bulb. Compared to one another, midday sunlight is more bluish,

    and incandescent light and sunlight around sunrise and sunset (during the so-called golden hours) are more yellowish.

    Fluorescent lights, LEDs and the xenon tubes in flashes produce light by spontaneous emission; fluorescent light is somewhat greenish and white LEDs often give bluish light; the light from a xenon flash is a close match for sunlight.

  • Black-Body Radiation

    This diagram provides a convenient excuse for the gratuitous use of the term black-body radiation and the ultraviolet catastrophe, which not only sounds really frickin cool, but would also make a totally rad name for a band

  • Tungsten vs. Daylight Tungsten, at around 2,500K, is about the lowest

    color temperature we encounter on a daily basis Sunlight, at around 6,000K, is one of the highest

    color temperatures we encounter

    White balance set for 2500K White balance set for 6000K

  • White Light & Color Temperature Thermal light sources give off a broad spectrum of frequencies

    (white light) characteristic of black-body radiation; Light from the Sun comes from its 6000K/10000F surface, the

    chromosphere, Incandescent light comes from the 2500K tungsten filament in

    a light bulb. Compared to one another, midday sunlight is more bluish,

    and incandescent light and sunlight around sunrise and sunset (during the so-called golden hours) are more yellowish.

    Fluorescent lights, LEDs and the xenon tubes in flashes produce light by spontaneous emission; fluorescent light is somewhat greenish and white LEDs often give bluish light; the light from a xenon flash is a close match for sunlight.

  • Correlated Color Temperature The black-body radiation spectrum

    describes the distribution of wavelengths in the ideal theoretical case

    In practice, real light sources have anomalous peaks and valleys in their light spectrum due to chemical impurities, filtration effects, etc

    Perceptually, there is an arbitrary number of spectra that more-or-less approximate a given ideal black-body spectrum

    A spectrum that does not conform to the ideal is assigned a so-called correlated color temperature The CIE 1931 x,y chromaticity space, also showing the

    chromaticities of black-body light sources of various temperatures (Planckian locus), and lines of constant

    correlated color temperature

  • When Blue is Hot and Red is Cold ...

    Be careful not to confuse color temperature with the psychological associations of blue with cold, and red with hot

    Bluish color temperatures are actually much higher than reddish ones - the opposite of our everyday associations

    If youve ever heated something until it gets white hot, you will have seen it go through red to orange to yellow before it reaches white heat - so red is the lowest color temperature you observed

    Photographic filter names perpetuate this confusion - a blue filter (like a Wratten 80 or 82) is called a cooling filter and an orange filter (like a Wratten 81 or 85) is called a warming filter even though the color temperature of the light that passes through them is shifted in the opposite physical sense

  • Tungsten vs. Daylight Tungsten, at around 2,500K, is about the lowest

    color temperature we encounter on a daily basis Sunlight, at around 6,000K, is one of the highest

    color temperatures we encounter

    White balance set for 2500K White balance set for 6000K

  • Flash - Lightning in a Bottle Photographic flash tubes, aka strobes, flash guns or

    speedlights(*) produce very intense light for a very short period (~1ms or less) by creating a high-voltage electrical discharge through a clear glass tube filled with xenon (or sometimes krypton) gas

    The first photographic flashes were created with magnesium powder, then later, strips of magnesium metal in a glass bulb, and then finally zirconium metal, which gave an even brighter light

    Light from a flash tube has a complex spectrum and a color temperature around 5600K, but for most purposes is a close analog for natural sunlight

    * Irritatingly, Nikon and Canon trademarked similar words for their battery-powered flashes. Nikon calls theirs the Speedlight, Canon, the Speedlite. Because of this, many photographers use the term in a generic sense for battery-powered portable flashes.

  • Colored Light Color is a sensation that results in the brain from the

    differing stimulus of the cone cells in the back of the eye; the color of an object is the result of the way it reflects and absorbs light of different colors; without light, there is no color

    Different wavelengths of light stimulate receptors in the retina that are sensitive to red, green and blue light to a greater or lesser degree - those nerve impulses are perceived in the brain as the sensation of color

    Humans have evolved to see best under bright white light, light that contains all the visible wavelengths more or less equally

  • Colors of Objects When white light hits an object, some colors of

    light are absorbed and some are reflected and scattered. For example, a green square reflects green light, and absorbs light of other colors. We see the green light reflected from the ball, so we perceive the square as green

    A black object absorbs light of all colors equally - which is why dark clothing feels hotter on a sunny day - and an object that reflects and scatters all colors of light will appear white

    If an object reflects all colors of light without scattering them it is said to have very high specular reflection - like a mirror

    A transparent object, like colored glass, transmits some colors of light and absorbs others. Objects we see as blue in white light will appear dark when a person (or a camera) sees them through a red filter. This is why colored filters are still useful in black & white photography

  • Mixing Colors

    The primary colors of light are red, green and blue. They mix together in an additive process - they add light to darkness. For example, red light + green light = yellow light

    The primary colors of ink, paint or dye are cyan, magenta, and yellow (plus black). Their colors mix together in a subtractive process - they subtract whiteness from paper. For example, magenta ink + yellow ink = red ink

    Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three are often used.

  • The Human Eye The back surface of the human eye is covered

    with a membrane called the retina, which is covered in light-sensitive cells called cone cells and rod cells

    Different cone cells are receptive to light of short, medium and long wavelengths, or, roughly speaking, blue, green and red light respectively. Cone cells need fairly strong light to work properly

    Rod cells are sensitive only to intensity of light, not color. They are responsible for our low-light vision.

    Over most of the retina, rod cells far outnumber cone cells, but the fovea, the highest-resolution area right in the center of the visual field, is populated almost entirely with cone cells

  • The Human Eye - The Tech Specs The human eye has an equivalent focal length of about 22mm, and the iris opens up

    to a maximum of about 7mm, for a maximum relative aperture of about f/3.2 in dim light, and closes down to a minimum of 1.8mm or about f/13 in bright sunlight

    In addition to the iris, the eye can also vary the sensitivity of the retina - the dark-adapted eye has a sensitivity equivalent to about ISO 800, the light-adapted eye has a much, much lower sensitivity - around ISO 1

    Overall dynamic range of the eye is about 8 orders of magnitude, instantaneous range is about 5 orders of magnitude - still roughly 10 times better than the best man-made sensor

    Even though the eye/brain does not perceive all the details from every pixel of the retina at every moment, the overall equivalent spatial resolution of the eye is in the hundreds of megapixels

    Most people can tell the difference between a single line and a pair of lines when those lines are spaced about 0.59 arc minutes apart (about 1/100) - just sufficient to resolve the crescent of Venus, which makes an image about 5m across on the retina

    The human field of vision approaches 180 in the horizontal and vertical dimensions

  • Human Color Vision Color is the way the brain perceives

    light rays of different wavelengths Our eyes do not contain separate

    receptor cells for every possible color; what the eye receives is a so-called tristimulus - the relative stimulation of the three different types of cone cells in the retina with different sensitivities

    Humans have three types of cone cell, but most birds have four, while pigeons may have five. Many mammals only have two, and marine mammals have only one, while marine crustaceans calls Stomatopods have twelve!

    The human eye contains roughly 64% red cones, 32% green cones, and 4% blue cones

  • Color & Human Perception

    Human eyes detect color using a trichromatic process, resulting in a tristimulus. Because the three types of cone cells overlap in their response to colors of light, it is more efficient for the visual system to process the differences between the signals

    In this way, although the eye effectively sees red, green and blue, the brains visual processing seems to work in terms of red-green bias, yellow-blue bias, and brightness (the so-called opponent process)

    When we perceive certain colors, eg, some shades of yellow, they might be coming from a single yellow light source, or it might be the combined output of red and green light sources, and the eye cannot tell the difference. This is called metamerism.

  • Color By Numbers The human eye sees color with receptors that sense

    short, medium and long wavelengths - roughly corresponding to blue, green and red primary colors of light respectively

    The words for colors - blue, green, red, yellow, purple, violet, crimson, scarlet, magenta and so on -are all subjective. For scientific precision, we need to use a numerical representation

    Taking the so-called Red/Green/Blue tristimulus model as our starting point, we can represent the intensities of these three primary colors as numbers, we can represent colors of light numerically - we call this scheme RGB

  • RGB Digital Color The range 0 - 255 is a convenient range for computers to

    represent in one byte, so one common way to represent colors in digital devices (like computers, and digital cameras) is as a set of three numbers. By convention, the first represents the amount of red, the second, green, and the third, blue. Higher numbers usually represent brighter light.

    In this way, (255, 0, 0) represents pure red, (0, 255, 0) represents pure green, (255, 255, 0) represents a bright yellow, (128, 128, 128) represents a mid gray, and so on.

    By using three sets of numbers in the range 0-255, we can theoretically represent more than 16 million separate individual colors. The eye is thought to be sensitive to 10 million or so different shades.

  • RGB Color Space Because the three numbers for Red, Green and Blue are

    independent of one another, we can use them they like the X, Y and Z coordinates in a three-dimensional space

    The resulting model is known as an RGB color space Used for monitors, cameras, scanners - additive color devices

  • L*a*b Color Space We can also choose to represent color

    using a variant of the brains opponent process. We can take the RGB values and mathematically transform them to represent them instead as lightness (L), red-green bias (a) and blue-yellow bias (b), or L*a*b

    L*a*b can also written Lab or LAB (and always pronounced ell-ay-bee, not like the word lab)

    Used for monitors, cameras, scanners - additive color devices

  • HSV/HSL Color Space We can use other mathematical transformations

    to arrive at two other closely related schemes known as HSV or HSL, standing for Hue, Saturation, and Value or Lightness

    It is sometimes preferable in working with art materials, digitized images, or other media, to use the HSV or HSL color model because of differences in the ways the models emulate how humans perceive color

    Graphical depiction of HSV

    HSL arranged as a double cone

    Comparison of the HSL and HSV color spaces

  • CMYK Color Space The colors of inks, paints and dyes mix

    in a subtractive model, and the primaries for printing are cyan, magenta and yellow, so in these cases we usually use a CMY model.

    In practice, just mixing cyan, magenta and yellow ink does not give a very good black, so we use a fourth number to represent the amount of black (or key) ink that is used, making the real model CMYK (since using B for black might be confused with B for Blue, we use K for key or blacK)

  • Lenses A lens is an optical

    device that transmits and refracts light, and can be used to form an image

    In the human eye, or in a camera, the lens and cornea gather and bend light rays so that they form an image on the surface of the retina

    In a camera, the surface is typically a viewfinder screen, or photographic film, or a digital sensor

    Lenses and lens systems are covered in much more detail in the Focus on Lenses module

  • Light Control Many lens systems employ a

    diaphragm that creates a variable-size aperture or to vary the amount of light that is allowed through the lens. This also controls depth of field

    In camera lenses, this is usually referred to as an aperture stop, and the usual design is an iris diaphragm

    In human eyes, it is called the iris. The pupil is name for the aperture or hole in the middle.

  • Anatomy of the Camera

  • Cameras All would-be photographers already possesses at least one camera -

    the human eye The word camera comes from Latin, from the same root as

    chamber as in room, and refers to a light-tight box with an opening at one end for the light-bending mechanism and a screen at the other end onto which the image is projected

    In the human eye, the lens and cornea work together to gather and refract light to form an image on the retina

    In most cameras, the cameras lens system gathers and refracts light to form an image on the film or sensor

    With a pinhole camera, light diffracts around the edges of a small hole instead refracting in a lens, and the image is usually projected onto a ground-glass or translucent paper screen

  • Types of Camera All cameras need a way to show the

    photographer the image the camera will take - eg a viewfinder

    The mechanism used in the viewfinder is one major classification of cameras

    View camera Rangefinder Twin-lens reflex Single-lens reflex Electronic viewfinder

  • Types of Camera Cameras are also classified according to the kind of

    sensor Film (silver halide) Digital (CCD or CMOS sensor)

    ... and according to what kind of image they capture Still images Moving images (video)

    ... and even sometimes by the purpose they are designed for

    Underwater High speed Astrophotography/Night vision/Infrared/

    Ultraviolet/Gamma/X-Ray/T-Ray

  • Compact vs. SLR Compact cameras, also known as point-and-shoots or P&S,

    are, as the name implies, physically quite small and almost always have a zoom lens that is not interchangeable

    Cheaper than most SLRs, partly due to small sensor size, which implies small lens size, which makes lens cheaper

    Small sensor size limits image quality Low price also means slower computer circuitry - slower to

    react, can be frustrating to use - missed shots, etc Almost all offer live image view on the rear LCD screen, some

    dont even have an optical viewfinder Can still pack a lot of sophistication inside, very convenient,

    quite economical

  • Why SLRs Arent Compact Historically, most SLRs are based on 35mm film, and modern digital SLRs

    usually fit in the same physical size range This is mainly to retain compatibility with lenses and other accessories for

    35mm film cameras Most digital SLRs have a sensor quite a bit smaller than a 35mm film frame

    One frame of 35mm film is 36mm x 24mm Most digital SLRs have an APS-C sized sensor about 24mm x 16mm This is the source of the so-called 1.5x/1.6x crop factor for lenses on a

    digital camera - some lenses, eg Canon EF-S etc are lenses made especially for APS-C digital SLRs, and cant be used on full-frame cameras

    A few digital SLRs exist with an APS-H size sensor with a 1.3x conversion Some full-frame digital SLRs exist, but they are expensive because its very

    costly to produce such a large sensor Olympus Four-Thirds system of SLRs and lenses is based on the APS-C size

    sensor and the rest of the camera system is sized accordingly

  • Advantage: SLR Bigger sensor = better quality = bigger lenses =

    higher prices Higher prices = more processor power = faster

    reaction time Bigger camera body = larger battery = more

    electrical power Physically larger lenses allow for direct manual

    control of zoom and focus Automation is fine so long as it can be overridden

    and prevented from getting in the way ...

  • Inside the SLR1. Interchangeable front-mount lens (4-

    element Tessar design)

    2. Reflex mirror at 45-degree angle

    3. Focal plane shutter

    4. Film or sensor

    5. Focusing screen

    6. Condenser lens

    7. Optical glass pentaprism (or pentamirror)

    8. Eyepiece (can have diopter correction ability)

  • Lenses Almost all cameras have a lens system to gather and focus

    light (or other parts of the EM spectrum) onto a sensitive medium (film or digital sensor)

    X-ray & gamma cameras usually work a little differently ...

    A pinhole cameras uses a small hole as a lens that relies upon diffraction effects rather than refraction but otherwise operates similarly

    One of the defining characteristics of a serious camera is having interchangeable lenses

    Much more information on lenses is covered in the module Focus on Lenses

  • Photographic Film Image-registration media based on light-

    sensitive chemicals have been around since the dawn of photography

    Earliest media were paper sheets or glass plates coated with light-sensitive chemicals; the earliest flexible plastic (camphor-plasticized nitrocellulose) film dates from 1889

    Photographic film is available in black & white, and color versions, and comes in two different basic forms - print film, and color reversal film, also known as slide or transparency film

  • How Film Works Although the details differ, they all work in essentially

    the same way - sensitized silver halide emulsion is briefly exposed to light (in the camera), then processed with chemicals that turn the latent image captured in the exposed silver halide into metallic silver. Color film goes through a number of other steps, some of which are repeated, to remove the silver and leave behind the colored dyes that makes up the image

    Print film, once it has been developed, can be used to project a negative image onto photosensitive paper - the photosensitive paper is then developed to reveal the final image, using a process that is similar to the one used to develop the film in the first place

  • Early Digital Sensors First digital sensors were proposed by Eugene Lally at JPL in 1961;

    the first practical digital imaging array - the CCD - was invented at Bell Labs by Willard Boyle and George Smith in 1969

    Early digital sensors relied upon the photoelectric effect, Einsteins 1905 description of which resulted in his receiving the Nobel prize in 1921

    The photoelectric effect is part of quantum theory, and Einsteins key insight was that light arrives in packets or quanta

    Each individual wavelength of light has a characteristic energy (given by Plancks law, e = h/!) - the photoelectric effect turns these light quanta or photons into electrons with a specific voltage. This is how early video imaging sensors worked - color value was translated into a voltage, and intensity (or photon flux) manifested as current (or electron flux)

  • How Modern Digital Sensors See Color

    As noted before, early digital imaging sensors exploited the photoelectric effect to directly read both average wavelength and intensity of light falling on each pixel

    In order to improve chip yield, improve robustness, lower cost and enhance performance, modern sensors, known as active pixel sensors, rely upon photoconductive effects to register light intensity, and per-pixel colored filters (known as the Bayer filter) to discriminate light of different wavelengths (aka colors). Each pixel has its own amplifier.

  • Please Look After This Bayer Dr. Bryce Bayer's patent used twice as many green elements as red or blue to

    mimic the human eye's greater resolving power with green light.

    As land animals surrounded by greenery, vision in humans evolved the ability to extract the most information from the green/red part of the spectrum because this was most crucial for survival - apart from the sea and the sky, there isnt a whole lot of blue in nature.

    The eye contains roughly 64% red cones, 32% green cones, and 4% blue cones

    These elements are referred to as sensor elements, sensels, pixel sensors, or simply pixels; sample values sensed by them, after interpolation, become image pixels

    Some (Sony) sensors actually have elements that register four different colors - red, green, blue and sapphire, a greenish-blue, to increase their ability to discriminate colors in the part of the visible spectrum in which human vision has the greatest sensitivity.

  • RAW Results

    The raw output of Bayer-filter cameras is referred to as a Bayer pattern image. Since each pixel is filtered to record only one of three colors, the data from each pixel cannot fully determine color on its own. To obtain a full-color image, various demosaicing algorithms can be used to interpolate a set of complete red, green, and blue values for each point

    Demosaicing is usually done in the cameras processor before it records a JPEG image; some cameras can be set to record the raw sensor data without demosaicing, requiring this to be done externally later - this is the so-called RAW imagedata mode that some cameras offer

  • Dr Bayers Dirty Little Secret A 12 megapixel camera might be advertised as giving images

    with 4,000 x 3,000 pixels Sure enough, the sensor has 4,000 x 3,000 (effective) sensor pixels But due to the presence of the Bayer filter, half of those sensor pixels

    are only sensitive to green, a quarter to red, and a quarter to blue In practice, then, the luminance (light/dark) spatial resolution of

    the sensor closely approximates the 12MP claim, but the chrominance (color) spatial resolution is only about a quarter - the rest is made up by interpolation

    This, however, mimics the human eyes configuration of rod and cone cells - the eye has many more rod cells that are only sensitive to luminance (and are well suited to night vision) than cone cells, that register color. The cone cells, however, need more light - this is why human low-light vision is largely devoid of color information.

  • Resolving Resolution Two important concepts are wrapped up in the

    same word: resolution The word resolution is, confusingly, used to

    describe both pixel density and total number of pixels

    Same concept applies to input devices (like camera sensors and scanners) as well as output devices (like printers and screens or monitors)

    For cameras, the link between these two things is sensor size

  • Output Resolution Most computer monitors today display between 70-100 dots per

    inch (or dpi) - limited by current manufacturing technology - with an average of 72dpi - where each dot or pixel is made up of 3 sub-pixels - one red, one green, and one blue

    Most printers have an addressable resolution around 300-360 dpi When inkjet printer manufacturers talk about 4800dpi or

    9600dpi, they are talking about the smallest droplets of ink that can be produced - but many ink droplets are sprayed out to make up one image pixel

    Commercial offset lithographic printers have used 240dpi for years as maximum worthwhile resolution

    Not worth exceeding this limit because the human eye cannot resolve beyond this limit

  • Sensor Size Silliness Digital camera sensors come in many sizes

    Almost all digital SLR sensors come in one of two sizes - full-frame or APS-C

    Compact camera & cameraphone sensors come in a huge range of sizes

    A small sensor with a large number of pixels = small sensor pixels

    Small sensor pixels = more noise See also discussion of circle of confusion in the Focus

    on Lenses module for practical limit on smallest sensor pixel size

  • Sensor Size & Crop Factor One frame of 35mm film is 36mm x 24mm Most digital SLRs have an APS-C sized

    sensor about 24mm x 16mm This is the source of the so-called 1.5x/1.6x

    crop factor for lenses on a digital camera - some lenses, eg Canon EF-S etc are lenses made especially for APS-C digital SLRs, and cant be used on full-frame cameras

    Nikons APS-C (DX-format) sensors are actually 24mm x 16mm (1.5x) and Canons are 22.5mm x 15mm (1.6x)

    A few digital SLRs exist with an APS-H size sensor with a 1.3x conversion

    Some full-frame digital SLRs exist, but they are expensive because its very costly to produce such a large sensor

  • Megapixel Madness Sensor pixel density is already approaching the ability or

    exceeding the optical capabilities of lenses Ultimately due to laws of physics, but also limited by defects &

    limitations of real-world materials & manufacturing techniques For a full-frame 35mm sensor, probably ~25MP, and for

    APS-C , probably~15MP Harder to say for compacts, but probably ~10MP for largest

    compact sensors At a certain point, adding more pixels to the sensor does not

    result in more usable resolution in the picture - less sharpness per pixel

    Smaller pixels also result in more noise due to photon counting statistics

  • Why Sensor Size Matters We have become accustomed to thinking that in the world of

    electronics, smaller is always better Not true with digital image sensors In all but the very dimmest light, the major source of noise in all

    modern digital cameras is photon counting statistics (photon noise) CCD and CMOS sensors can hold about 800 to 1600 electrons per

    square micron (photon to electron ratio need not be 1:1 due to gain in the sensor circuitry - this is how ISO speed rating is varied in a digital camera sensor)

    Photon noise is proportional to the square root of the number of photons captured in each well

    All other things being equal, larger sensor pixels give better dynamic range, better sensitivity, lower noise, and finer tonal gradations

  • Film Redux Despite all the obvious advantages of digital,

    photographic film still has the lead over digital sensors in two places:

    The ultimate limit on resolution (although for most practical purposes this is largely irrelevant) and

    The ultimate limit on ability to capture dynamic lighting range (five or more orders of magnitude for the best film versus four or so for the best digital sensors), although this too is beginning to change, especially with techniques like HDR

  • Aperture, Time & Sensitivity

  • Getting a Suntan

    There are three things to bear in mind when tanning: How strong is the sun? How long should I expose

    my skin? How sensitive is my skin?

    Getting these wrong can have painful results

  • Taking a Picture

    There are three things to bear in mind when taking a picture: How bright is the light? How long should I expose

    the sensor? How sensitive is the sensor?

    Getting these wrong can give ugly results

  • Getting a Suntan - Redux How do I control the three factors of tanning?

    Choose when and where to tan - strength of light varies with latitude, season, time of day, cloud cover, surroundings, and the size of the hole in the ozone layer!

    I go inside when Im done I can make myself less sensitive to the sun with a base tan

    Getting these right can have great results I can also modify my environment, eg, by adding or

    substituting UV tanning lamps, or by being under a fabric cover that reduces the intensity of the UV or apply sunscreen

  • Taking a Picture - Redux

    How do I control the three factors of exposure? Vary the size of the aperture Vary the shutter speed Vary the ISO sensitivity

    Getting these right can have pleasing results I can also modify the environment, eg, by

    adding more light with a flash, or by putting a neutral density filter in front of the lens

  • Exposure by the Numbers The three main factors that we can vary in setting an exposure on a

    camera - aperture, time, and sensitivity - are measured in different units Aperture is measured in f numbers or f stops

    Typical range is f/2 to f/32 Time is measured in seconds, or fractions of a second

    Typical range is 30 - 1/2000 Sensitivity is measured in ISO units

    Typical range is 100-1600 ISO The range of lighting conditions we encounter vary by a factor of a

    million or more between a very dark room, and a bright sunny day Photographers cope with this by using units that halve or double their

    value between steps (logarithmic scales) - especially with f numbers

  • f Numbers and Aperture f numbers can be a little confusing ... The smaller the number, the larger the aperture, the

    more light is let in, and the shallower the depth of field

    f numbers - also known as relative aperture - are the ratio of the lenss focal length to its aperture size

    If the focal length stays the same, and the aperture size gets smaller, the f number goes up

    If the focal length gets longer, and the aperture size stays the same size, the f number goes up

  • f Numbers in Practice The laws of physics limit the largest possible aperture to about f/0.5 - the

    movie Barry Lyndon was shot with an amazing NASA-developed f/0.7 lens that allowed the director to shoot the interior scenes by candlelight

    Zeiss and Canon have both made and sold 50mm f/0.95 lenses Lenses with a large maximum aperture ( = small f number) tend to be

    very large in diameter, made from big pieces of optical glass, and are therefore expensive

    The iris of the human eye opens up to a maximum relative aperture of about f/3.2 in dim light, and closes down to a minimum of about f/13 in bright sunlight

    By convention, the typical step between f numbers is the square root of 2 - roughly 1.4x. Each step between f numbers represents a doubling or halving of the amount of light. The actual figures are typically rounded up or down for convenience.

    f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32 ...

  • Aperture and Depth of Field The depth of field (DOF) is the

    portion of a scene that appears sharp in the image.

    Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on either side of the focused distance, so that within the DOF - or zone of acceptable sharpness - the image appears sharp

    A wide aperture (small aperture number) results in a shallow depth of field, a small aperture gives more depth of field

    Flowers at f/5.6

    Flowers at f/32

  • Automatic Aperture In most cameras, the aperture of the lens remains wide

    open most of the time, only stopping it down to the set value right before a picture is actually being taken, and then opening it up again right after

    This is done to give the user a nice, bright viewfinder, and to make critical focusing easier

    Most digital SLRs have a button that will make the aperture close down to the preset value so you can see the depth of field you will actually get when the picture is taken (you will also see the view in the viewfinder get darker)

    This button is usually labeled depth-of-field preview for that reason

  • Shutter Speeds Probably the easiest of the three concepts to grasp - how long does the

    shutter stay open to expose the film? The way the camera displays shutter speed can be confusing - a

    display of 8 means eight seconds, but a display of 8 means one eighth of a second

    Can be as much as 30 or longer (for night-time, astrophotography, or other special effects)

    Can be as little as 1/8000th of a second with a mechanical shutter - special-purpose high-speed cameras with electronic shutters can expose for as little as 1/2,000,000,000th (half a billionth) of a second

    High shutter speeds help to freeze fast-moving action and reduce or eliminate the effects of camera shake

    Shutter speed steps traditionally went in steps of doubling or halving 1, 1/2, 1/4, 1/8, 1/16, 1/30, 1/60, 1/125, 1/250, 1/500 ...

  • How Does The Shutter Work? Some cameras use a shutter mechanism inside the lens,

    very similar to the iris or aperture, called a leaf shutter Most modern cameras instead employ a mechanical

    shutter that consists of two curtains that move across the focal plane (where the film or sensor is located) one after the other

    The first curtain moves across the focal plane and exposes it (shutter is open)

    Then the second curtain moves across the focal plane and covers it up again (shutter is closed)

    Curtain movement is usually spring- or motor-driven

  • Slow Shutter Speed (
  • Fast Shutter Speed (> X-Sync) First curtain starts moving across

    the focal plane Second curtain starts to move before

    first curtain has completed its traversal of the focal plane

    Shutter is never fully open, but instead an open slit travels across the focal plane

    Second curtain completes its traversal and the shutter is closed

  • Pay No Attention to the ManSensor Behind the Curtain

    In most modern cameras, the curtains travel in a vertical direction because that is the shorter dimension of the sensor (or film in older cameras)

    Still, the curtains take some finite period of time to travel - lets say 1/250s This is relatively insignificant if were talking about an exposure of 1

    second - but what if we want an exposure of 1/4000s? In practice, there is some shutter speed beyond which the curtain travel

    time exceeds the nominal period of time that the shutter is open - X-sync speed (q.v.)

    What happens then is that the second curtain starts to move before the first curtain has reached the other side - the result is a slit that travels across the focal plane.

    Even though it may still take 1/250s for the curtain to travel all the way across the focal plane, if the second curtain follows 1/4000s behind the first, then each and every point in the focal plane (on the sensor) will be exposed for 1/4000s

  • Implications of Slit Exposure Because of this behavior, there is some maximum

    shutter speed at which the shutter is fully open - sometimes as slow as 1/60s, but can be as fast as 1/500s (eg Nikon D70)

    Beyond this shutter speed, the sensor is only exposed in a slitwise fashion

    What if you take a picture of something moving very fast?

  • Sensitivity How sensitive the film or digital sensor is to light Measured in ISO units (same as the old ASA) - sensitive film -

    high ISO - is said to be fast, and insensitive film - low ISO - isslow

    Film for slides (transparencies) used to be available in speeds as slow as ISO 25, and the fastest photographic film commercially available was around ISO 3200

    Films effective sensitivity could be varied by processing it in different chemicals or for a different length of time (push or pull processing)

    Most digital cameras can vary their sensitivity from 100-1600 ISO via a dial or a menu setting

    Higher ISO sensitivities result in more noise in the image Again, typically vary by powers of 2

    100, 200, 400, 800, 1600 ...

    ISO 100 - f/16, 5

    ISO 3200 - f/16, 1/6

  • Exposure Cheat SheetEXPOSURE

    FILM

    SP

    EE

    D (I

    SO

    )

    SH

    UTT

    ER

    (SE

    C)

    AP

    ER

    TUR

    E-noise100 (L1)200400800160032006400 (H1)+noise

    light

    erda

    rker

    -motion blur1/40001/20001/10001/5001/2501/1251/601/301/151/81/41/212...bulb+motion blur

    (4000)(2000)(1000)(500)(250)(125)(60)(30)(15)(8)(4)(2)(1)(2)...(bulb)

    +doff22f16f11f8f5.6f4f2.8f2f1.4f1-dof

    Source: http://glark.org/media/exposure-cheat-sheet.pdf - Used with permission

  • The Exposure Triangle As mentioned before, between these three

    quantities, photographers have to adapt to an enormous range of lighting conditions

    We can change the amount of light recorded on the sensor by varying the aperture, the exposure time (shutter), and the sensitivity of the sensor

    Light --> Aperture --> Shutter --> Sensor We can arrange these three concepts at the

    corners of a triangle

  • The Exposure Triangle1s

    1/1000s

    Sensitivity (ISO)

    ApertureTime

    Grain

    BokehMotion Blur

    1s

    1/1000s f/32

    f/1

    1600

    100

  • The Exposure Triangle The triangle defines an exposure space How much of the triangle you can

    explore & still get acceptable exposures depends on how much light you have

    In very bright and very dim conditions, you have fewer choices for usable combinations of shutter, aperture, and sensitivity

  • Aperture, Time & Sensitivity We can talk about combinations of these parameters either as Light

    Values (LV) or Exposure Values (EV) LV0 is defined as the lighting conditions that require an exposure of

    1 second at f/1 with ISO 100 sensitivity EV0 is the exposure that results from opening the shutter for 1

    second at f/1 with ISO 100 sensitivity EV and LV are open-ended scales - values in everyday use vary

    from 0 to 18 or so - and each step represents a doubling or halving Higher LV values represent brighter conditions Higher EV values represent less light being let into the camera, for a

    shorter period of time, with less and less sensitivity To nominally expose a picture under LVx conditions, set the camera

    to EVx

  • LVs for Bright Conditions Scale is open-ended - but limited on the high end by how much light we can expect to

    encounter on Earth LV0 represents a very dark room LV3 might be a brightly-lit outdoor street scene at night LV7 is typical indoor lighting levels, or outdoors about 10 minutes after sunset LV10 is about what youd find on a dull, dreary, overcast day in London, Paris or New York LV13 is what youd find on a typical cloudy bright day LV14 is the light youd find on a nice, side-lit scene in good afternoon light LV16 is what youd register for Caucasian skin in full sunlight LV18 would represent bright sunlight reflecting off a shiny object or the sea The Sunny 16 rule of thumb says that on a bright, sunny, day (say, LV16), set the aperture

    to f/16, and shutter speed to (as near as you can) 1/ISO sensitivity, eg: ISO 100, f/16, shutter speed 1/125

    By extension, the same rule applies at other LVs - with f stop set to (roughly) f/LV and ISO sensitivity of x, set shutter to roughly 1/x

  • LVs for Dim Conditions Scale is open-ended - can go negative too LV0 represents a very dim room LV -5 represents a scene lit only by the full moon LV -15 represents a scene lit only by starlight (new moon) So-called photographic

    darkrooms are almost never completely dark, but somewhere around LV -15

    Even though our eyes do not see colors well by moonlight, theyre still there

    This moonlit scene was exposed at ISO 400 atf/4 for 480 seconds - roughly EV -7

  • Table of Common LVs

  • Useful iPhone Apps

    Expositor Exposure calculator Vary LV and ISO, f/

    stop or shutter to find matching EV

    $1.99

  • Useful iPhone Apps

    DOFMaster Depth of field

    calculator $1.99

  • Useful iPhone Apps

    Focalware Sun/moon rise/set/

    position calculator $4.99

  • Automation - Help or Hindrance?

  • Automatic for the People As with cars, guns and other mass-market gadgets, early examples

    were completely manual, utterly lacking in automation Early cameras did not even automatically time the exposure - you

    would use the lens cap as the shutter - which was fine when exposure times were often several minutes, even in bright sunlight, due to the comparative insensitivity of the photographic medium

    Exposure timing was the first thing to be automated in any way, and then exposure calculation, followed (much later) by auto film advance, auto-focus, and today, even rudimentary auto-composition (!)

    Anything automatic on a camera is best thought of as computer-assisted rather than fully automatic - and therefore requires:

    A sentient, aesthetic being to make the final decision, and A way to make manual adjustments and override the computers

    recommendation

  • Automatic for the People - Automatic Exposure

    Early light meters helped to take some of the guesswork out of calculating an exposure

    The first real automation came when a light meter was coupled to the cameras exposure controls - thus was born AE, or automatic exposure

    Early meters were hand-held or separate from the lens - through-the-lens or TTL metering came later

    Most of the following discussion concentrates on measuring exposure for continuous light sources (ambient light, daylight, etc), not flash

  • The AE Problem Remember our high-key and low-key images? How does an

    exposure meter know what key were aiming for? Answer: it doesnt. Even todays AE systems, no

    matter how intelligent, still assume youre shooting a picture of something that is 18% gray, meaning it reflects 18% of the light that falls on it, across the spectrum

    Result: If you try and shoot a black cat against a dark wall, the cameras AE system will try and render it as gray, and overexpose it

    Or, if you try and shoot a skier in light clothing against a snowy background, the AE system will still try and render it as gray, and underexpose it

    AE can also be thrown off by shooting contre jour (into the light), or if there is a very bright reflection or light source somewhere in the scene.

    18% Gray card

  • Alleviating AE Problems You can do one of several things:

    Take the picture anyway, and fix it later in Photoshop Set the exposure manually Use the cameras partial metering or spot metering mode (if it has

    one) to get an exposure reading (AE lock) for part of the scene (you choose), then recompose to take the picture

    Use the cameras exposure compensation control to tell it to under or over expose the picture, relative to what the AE system is telling you

    These are listed in order of least to most attractive options (generally) Exposure compensation is often set with a dial or a menu selection,

    and usually allows you to adjust the exposure up or down by 2 stops (EVs)

  • AE Metering Modes The default for most cameras is to take readings from across the

    entire scene but to give more precedence to readings from the center of the scene (so-called center-weighted average) or to give preference to some other areas (evaluative metering)

    Other modes may include partial metering - typically a subset of the whole scene, again biased towards the center - or spot metering - taking a reading only from the very center of the scene (as seen in the viewfinder)

    This discussion primarily applies to shooting in ambient or continuous light - flash metering is a much more complex topic

    Center-weighted metering Partial metering Spot metering

  • Exposure Modes They dont address the AE Problem as described

    on the previous slides, but most good cameras have at least four exposure modes:

    Aperture priority, Av Shutter priority, Tv Program AE, P Manual, M

    A stands for Aperture, T for Time, and v for variable - meaning thats the parameter you vary

  • What the AE Modes Do In Av mode, you set the aperture (and sensitivity) you want, and

    the cameras AE system will set an appropriate shutter speed In Tv mode, you set the shutter speed and sensitivity, the camera

    sets the aperture In Manual mode, you set everything manually. No surprises. In Program mode, everything is set for you, but may offer a

    Program shift function where you can turn a dial to select different combinations of shutter & aperture that give the same EV

    In any mode, you may still want to override the programmed behavior as what the camera computes to be the right exposure will always be a compromise, for the average situation (which is fine if you want to take average shots of 18% gray subjects)

  • Even More AE Modes Some cameras (eg Pentax K-7) also offer an Sv mode, where the S

    stands for Sensitivity - you set Sensitivity, it sets aperture & shutter Some cameras will also offer other modes like Sports or Action,

    Portrait, Macro, Theater, Landscape, or Night These modes are not fundamentally different than those above - they

    are usually based on Program mode, but may be biased towards selecting the fastest shutter speed possible, continuous shooting mode, the widest aperture, or the smallest aperture, while taking into account lighting conditions (white balance), focal length (zoom position) and so on. There is no magic!

    Your camera may also have an Automatic Exposure Bracketing mode - it will take multiple exposures (usually 3) with different exposure compensation amounts - usually up to +/- 2 stops. This was useful for its intended purpose in the days of film, when you couldnt see your results straight away - these days its useful mostly for HDR effects

  • Whats the Right Exposure? Deceptively simple question, no single right answer -

    subjective, judgment call based on aesthetic preference, composition, and intended effect

    The retina of the human eye has an enormously wide dynamic range of sensitivity (exposure latitude), but also uses the iris to control the amount of light

    Films exposure latitude is not as broad as that of the human retina, and for digital sensors, its even narrower

    Film exposure had the convenient property of shoulders - non-linear response - the more light fell on the film, the less sensitive it got, making it harder and harder to expose it more

  • Exposure is a Compromise The range of brightness in a scene often

    exceeds the range of your sensor Result: blown highlights and/or plugged

    lowlights (bright areas are featureless white, dark areas are featureless black)

    Maybe able to lift shadow areas with fill lights (eg flash) or a reflector, or mitigate brightest sections by blocking some of the light - reverse fill

  • Finding the Right Exposure One theory of correct exposure aims for greatest tonal

    range in the final picture, not necessarily what you see in the viewfinder or preview screen

    Assumes you will post-process the image Adjust exposure so the brightest parts of the image are

    almost, but not quite, blown out, then adjust in Photoshop with a curves layer if necessary - expose for highlights, process for shadows (like the Zone Systems advice for slide film, converse of its advice for negative film)

    A digital cameras histogram display (if it has one) is useful here

  • The Zone System A photographic technique for determining

    optimal film exposure and development, formulated by Ansel Adams and Fred Archer in 1941.

    His technique was to carefully study a scene, visualize the final print, then determine the correspondence between portions of the scene and tones in the print. He would then meter, expose and develop the negative accordingly.

  • The Zone System scale The basic concept is to look

    at a scene, determine the major elements of the scene, visualize how you want them to be rendered, and adjust the exposure & processing to place them on that zone

    Formalizes the process most experienced photographers probably use to some extent without even knowing it

  • Using the Histogram A graphical bar-chart display of tabulated frequencies In digital photography, shows how many pixels of each

    brightness level are in a scene Generally turned on via menu option Can be hard to judge if a picture is properly exposed by

    eyeballing it on the LCD preview screen - especially in bright light

    Camera may help by showing blown-out highlights & plugged shadows, but the histogram gives more information

    May have a choice between one histogram showing overall range of brightnesses, or three showing brightness for each channel - red, green and blue

  • What is a Histogram? A histogram graphically

    shows the distribution of different values of a variable

    On the x axis, discrete values or intervals are marked

    The y axis then shows the corresponding frequency or number of occurrences of that value or falling into that interval

  • Histogram Examples

    Darkest to lightest

    # of pixels

    Over-exposed (+2 EV) Under-exposed (-2 EV) Correctly exposed

    Can we do better?

  • High Dynamic Range (HDR)

    HDR image made by combining the three exposures shown previously, in Photomatix PRO

  • HDR - Tools and History Concepts originated in the 1850s with Gustave Le Gray,

    who combined different negatives to print one positive Further developed in the 1930s and 1940s by Charles

    Wyckoff who created special 3-layer film to capture the 3Ss - sunshine, shadow and subject - in a single exposure and produce, among other things, Life magazines iconic images of nuclear explosions

    Built-in to Photoshop CS3 and up as Exposure blending As a standalone program/Photoshop plugin as Photomatix

    Pro, Radiance, and others Open-source tools include Qtpfsgui HDR is a powerful technique, but its easy to overdo it

  • When HDR Goes Bad

    One of my early HDR experiments - over-saturated and unrealistic, nonetheless, the

    most popular image in my Flickr photostream :)

    Another somewhat over-dramatic HDR sky

  • The Future of HDR Subtle HDR can look completely natural, or like a painting I believe HDR is here to stay - too useful not to be - and already being

    built-in to some cameras, eg Ricoh CX1 that makes two successive captures at different EVs and combines them in-camera, Pentax K-7 will do it with three, and the Fuji FinePix S3 pro DSLR, which contains sensor elements of differing sensitivities

    Naturalistic HDR Painterly HDR

  • Flash Metering Flash metering can be a very complex topic in its own right Essentially, most SLRs treat flash exposure as two separate exposures -

    one for the background, one for the foreground - but this can vary by AE mode, as well as the arrangement of the scene youre photographing

    Background in this sense means the areas of the scene that are not lit by the flash, and foreground means the areas that are

    Exposing for the background is done as if there were no flash attached - camera reads the light from the scene, and sets aperture and/or shutter accordingly, depending on AE mode. Caution - this can result in a long exposure when shooting in dim light (aka dragging the shutter).

    Exposing for the foreground is done by setting off a pre-flash and measuring how much light is reflected back to the camera - this information is then used in conjunction with aperture information to set the flash output

  • Flash Exposure Made Easy

    Set everything to P, Program or Auto, let the camera worry about it

    May get the results you desire or expect, or you may not ...

  • Flash Exposure Made Hard Set camera to Av or M and, with the flash turned off, set an exposure that

    gives you the background looking the way you want it to. Then turn the flash on, and take a test shot - adjust the flash exposure to get the foreground lit the way you want

    Shutter priority (Tv) not really a very useful way to set exposure with flash as we shall see in a moment

    Things to consider: Are you shooting a fairly bright scene, and you just want to add some

    fill light? Or are you shooting a dim scene, and the flash will be your primary

    light source? Is there a well-defined background vs. foreground, or is it all

    foreground? Or all background? Is the foreground highly reflective? How far is it from the flash to the subject (flash-to-subject distance)

  • Why Is Flash So Hard? Basically, because there are often two separate exposures to consider

    - foreground and background Examples:

    Night shot, outdoors, person standing fairly close to camera in an open field

    Daytime, outdoors, person standing fairly close to camera in an open field

    Indoors, low ambient light, person standing 5 feet from camera, in front of a wall 20 feet behind them lit by wall-washers

    Indoors, brightly-lit art gallery, standing in shadow, in front of a huge painting, 20 feet away

    Setting the right exposure(s) for foreground and/or background are largely a matter of artistic choice

  • Flash Exposure Example Consider example 3 from before:

    Indoors, low ambient light, person standing 5 feet from camera, in front of a wall 20 feet behind them lit by wall-washers

    Lets say you want both in focus - this implies a small aperture (so less light coming into lens) - and both evenly lit

    Small aperture implies high sensitivity and/or long exposure for correct background exposure (long exposure combined with flash is known as dragging the shutter)

    Long exposure makes camera shake more likely, and can lead to blur due to subject movement

  • Flash Exposure Example As long as it is slower than X-sync speed, shutter speed has no bearing

    on flash exposure - aperture, however, does affect flash exposure - but will also affect background exposure

    Small aperture will require more output from flash to illuminate foreground - may or may not be enough to light background, too

    More complex still if foreground person is dressed in black, and wall is white, or vice versa ...

    Shutter priority complicates situation even further - aperture affects depth of field as well as foreground (flash-lit) & background (non flash-lit) exposure, so best to keep it under your control

    In a typical setting, I leave my shutter at 1/250s to minimize camera shake and subject motion blur, vary the aperture & sensitivity to get the degree of ambient lighting & DOF I want, and rely on the flashs TTL metering and flash exposure compensation to correctly expose the foreground subject

  • Bright Flash, Dim Flash? You cant easily regulate the intensity of a flash tubes flash, but you can quite easily

    vary its duration. This is how flash tube power output is effectively regulated. The fast pulse of light from a flash tube can be used to freeze motion - a large

    studio flash at full power may produce light for as long as 1/400s, but a small battery flash may produce a flash as brief as 1/30-40,000s (25-33 sec) on its lowest power

    High-speed photography need not rely on fast shutter speeds, but rather short, intense flashes of light. This was taken 1/250s @ f/16, Canon 580 EX II flash @ 1/64 power, giving a measured flash duration of about 1/31,000s

    Flash output does varies throughout the duration of the flash pulse due to physical factors. There is typically a fast ramp-upto full output, then a gradual decay in light intensity, followed by an abrupt drop-off at the end of the flash.

    At full power, the 580s flash is 1200s. If its output were linear, then 1/64 power would be around ( 1200 64 = ) 19s - in fact, its about 32s because the flash tube is still ramping up

  • Automatic for the People - Automatic White Balance

    Thermal light sources give off a broad spectrum of frequencies (white light) characteristic of black-body radiation; light from the Sun comes from its 6000K/10000F surface, the chromosphere, and incandescent light from the 2500K tungsten filament in a regular light bulb. Compared to one another, midday sunlight is more bluish, and incandescent light and sunlight around sunrise and sunset (during the so-called golden hours) are more yellowish.

    Fluorescent lights, LEDs and the xenon tubes in flashes produce light by spontaneous emission; compared to sunlight & incandescent light; fluorescent light is somewhat greenish and white LEDs often give bluish light compared to midday sunlight, whereas the light from a xenon flash is a good match for sunlight.

    All digital cameras have an automatic white balance setting - some are better than others ...

  • Which White Balance? Most digital cameras can be adjusted for white balance or WB to take these

    differences into account. In the old days, you would buy tungsten-balanced or daylight-balanced film and/or use correction or conversion filters on the lens to get the right white color

    Many cameras will also have an AWB for automatic white balance mode. They will take an educated guess but can get it wrong - again, theres no magic.

    White balance set for 2500K White balance set for 6000K

  • Flickering Fluorescents Fluorescent lights also flicker. If your shutter speed is < 1/60s,

    you can get weird color shifts, dark bands, etc. LEDs will also flicker unless they are powered from a clean DC source.

    1/30 second - I caught two complete cycles of 60Hz AC with this one - the

    illumination is even

    1/320 second - above X-sync speed - light was turning on or off as the

    shutter was traveling

    1/100 second - not so lucky this time - light was almost completely off

    while the shutter was open

  • White Balance Problems What if I set the wrong white balance?

    Fixing it later is no problem in RAW; only slightly harder with JPEG What if I have a mix of light sources? Eg daylight, fluorescent & tungsten?

    Choose the dominant light source and go with that Turn off or mitigate the most problematic source(s) - usually fluorescent Close the blinds, turn off the fluorescents, turn up the tungstens, gel

    your flash Shoot in B&W

    What if Im at a rock concert, or shooting a play at a theater? Tungsten is probably your best bet

    What if its a weird light source my camera doesnt know about, like HMIs? If you camera can do it, set a custom white balance using a gray card,

    otherwise set it to whatever looks nearest, and fix it later if necessary

  • Automatic for the People - Automatic Focus

    Automatic focus is a much tougher problem than automatic exposure

    Early systems were often active, and would bounce infra-red or ultrasonic beams off the subject to determine their distance from the camera - easily fooled and often inaccurate

    Early passive systems required the photographer to focus the lens manually, and just gave a confirmation when the system judged the focus to be correct

    Later, camera manufacturers introduced motorized focus adjustment, and created a feedback loop between the passive AF detection circuitry and the AF motor

  • History of Auto-Focus Between 1960 and 1973, Leica patented a variety of autofocus and corresponding

    sensor technologies, and in 1978 they displayed an SLR camera with fully operational autofocus.

    The first mass-produced autofocus camera was the Konica C35 AF, a simple point and shoot model released in 1977.

    The Polaroid SX-70 was the first autofocus single-lens reflex camera, released in 1978. The Pentax ME-F, which used focus sensors in the camera body coupled with a

    motorized lens, became the first autofocus 35mm SLR in 1981 In 1983 Nikon released the F3AF, their first autofocus camera, which was based on a

    similar concept to the ME-F The Minolta Maxxum 7000, released in 1985, was the first SLR with an integrated

    autofocus system, meaning both the AF sensors and the drive motor were housed in the camera body, as well as an integrated film advance winder - which became standard configuration for Minolta (now Sony), and Nikon

    Canon, however, elected to develop their EOS (Electro-Optical System) system with motorized lenses instead.

  • The AF Problem The sharpest image is a matter of

    interpretation, and there are different definitions, and methods to determine when it has been found - eg highest contrast

    Which part of the image should be sharpest? Some subjects like things with close vertical

    stripes or low contrast or that are dimly lit often present problems for AF systems

    Again, there is no magic.

  • Alleviating the AF Problem Most AF systems are biased towards subjects in the

    center of the viewfinder Thats fine as long as your item of interest is in the center Most systems allow you to press the shutter half way to

    get AF lock on the subject in the center of the viewfinder, then recompose while holding the button halfway, then press the shutter all the way to take the picture

    Dim lighting can also be a problem - mounting a flash on your camera may give it an AF assist beam. If you are taking a picture in deliberately dim lighting, turn the light up to get AF lock, then turn it back down to take the picture

  • Why Are My Pictures Blurred? Not all blurring is focus-related If you go to see a doctor and you say you are suffering from

    dizziness, your doctor will say - which do you feel ... Off-balance? Vertigo? Pre-syncope? (About to pass out)

    Similarly, blurriness can have a number of causes ... Images can appear soft, they can be streaky, they can just lack

    detail ... Two basic causes of blurriness:

    Movement of subject or camera Optical blur either from incorrect focus, or diffraction effects

  • How Do I Avoid Movement Blur? Camera movement can be minimized with a tripod

    or mitigated with image stabilization Subject movement can be minimized by using a high

    enough shutter speed Using a flash can help to freeze subject movement,

    but dragging the shutter can result in combinations of blurred & sharp elements - something you might exploit for artistic effect

    You can also choose to embrace subject movement & blur and make it part of your composition with a panning shot

  • How Do I Avoid Optical Blur? Make sure your lens is clean! Use a high-quality lens tissue or cloth If youre using AF, focus on your subject in center-frame then recompose

    holding the shutter button halfway down Make sure youre not too close to your subject (ie not within your lenss

    closest focus distance) Make sure all parts of the subject that you want to be in focus are within the

    lenss zone of acceptable sharpness or DOF - basically, use a small enough aperture - check by stopping down the lens before releasing the shutter

    If you use too small an aperture, your lens will suffer from diffraction effects, which also reduce detail/resolution

    Most lens systems are sharpest around f/8 Background blur (or bokeh) definitely has its place in photographic

    composition - and the deliberate use of blurriness can create beautiful abstract shots

  • Can I Fix Blurriness Later? Unlike with slightly-off exposure, which can usually be corrected in post-

    production with Photoshop, you cant REALLY fix a problem with incorrect focus.

    Focus problems actually result in a loss of information, or an increase in ambiguity in the image data - and you cannot, in general, recover lost information.

    Sharpening filters in Photoshop can improve the apparent detail in an image by increasing local contrast around edges, but this is not the same thing as sharper focus.

    Similarly, complex deconvolution software like FocusMagic can help improve the appearance of out-of-focus images, and images with motion blur due to camera movement, particularly when the error is small in magnitude, but there will always be problems that cannot be fixed after the fact. FocusMagic can help recover information for forensic purposes even when the result is not aesthetically pleasing.

    Is there magic? No, there is no magic, despite what you might see on CSI or 24

  • Manual Focus Even the best AF system in the world, under ideal conditions, wont always

    give you the best possible focus - particularly with tricky or moving subjects, or if youre trying to focus a telephoto lens through a shimmering summer heat haze

    You will have to resort to manual focus in these situations. Sometimes this is selected via a switch on the lens, other times its a menu option

    Sometimes you may even have to line up, compose, get an AE reading AND focus for the subject before it comes into view - such as with fast-moving cars on a track. This is called preset focusing, and can work well if your timing is good.

    If you have a live view feature on your camera, this can really help you get spot-on focusing with a fast (wide-aperture, shallow DOF) lens, especially when the camera is mounted on a tripod. If you can magnify the live view image, you can even get the same kind of dead-on focusing advantage that used to be afforded to a photographer with a large-format view camera and a magnifying loupe

  • Focus on Lenses

  • Focal Length We are familiar with the idea that photographers use long lenses to

    bring distant objects close - like with a telescope Similarly, lenses that take in a wild field of view tend to be short and

    squat The main property that varies between these two types of lens is their

    focal length, or magnification power The focal length of lens is, roughly, defined as the distance between

    the lens and the image that it forms, when it is focused on an object that is infinitely far away

    The human eye takes in a field of view of almost 180, and has a focal length equal to the size of the eyeball from front to back

    Intuitively, we can think of a long (focal-length) lens as having a narrow field of view & high magnification, and a short (focal-length) lens as having a wide field of view, and low magnification

  • All I Wanna Do is Zoom-a-Zoom-Zoom-Zoom and a Poom-Poom

    A zoom lens is one with a variable focal length (also referred to as variable magnification)

    Principles first described in the Proceedings of the Royal Society in 1834, for telescope applications

    Zooms are optically and mechanically more complex than primes

    Also heavier & more expensive with more optical compromises - but very flexible

  • Zoom Ratios Ratio of longest to shortest focal length for a zoom lens is called its zoom ratio E.g. - a 17-85mm zoom has a 5:1 zoom ratio The larger the zoom ratio, the more compromises are made in the design -

    even today, the best zoom lenses can only match the performance of an equivalent prime lens up to about a 3:1 zoom ratio

    The highest zoom ratio available in an SLR lens today is in the 15:1 range - eg 18-270mm - although there are so-called superzoom compact digital cameras with zoom ratios as high as 26:1 where the quality demands are not as high

    The highest zoom ratio available today in any lens is probably the Panavision 7-2100mm - a video lens with a 300:1 zoom ratio

    Max aperture @ 7mm = f/1.9, @ 2100mm = f/13 Optical aberrations and distortions are not as noticeable on video as they

    are with stills. Also, even an HD video camera is limited to fewer than 2 megapixels, so their resolution is low relative to todays digital still cameras

  • Plus a Change: Zoom and Aperture

    Remember our definition of f number? If the focal length increases but the iris remains the same size, the resulting f number will get bigger and the relative aperture gets smaller

    E.g. - a 17-85mm zoom might open up to f/4 @ 17mm focal length, but only up to f/5.6 at the 85mm focal length

    Most zoom lenses have this issue although zoom lenses do exist with constant (relative) aperture eg Canon has a 70-200mm zoom with a constant f/2.8 maximum relative aperture

  • Plus a Change: Zoom and Focus Before the days of AF, it was important for a zoom lens to maintain

    the same focal distance when the focal length was varied - such lenses were said to be parfocal and were more complex and more expensive than non-parfocal zoom lenses (strictly speaking, a non-parfocal zoom lens is called a varifocal lens)

    Advice used to be given to zoom in all the way to focus, then zoom out to frame the shot & take the picture

    This advice only applied to parfocal lenses and is actually counterproductive for non-parfocal lenses

    Since AF makes (re)establishing focus so easy at any focal length, parfocal behavior is no longer necessary

    Cameras equipped with live view often allow magnification of the live image - this can help the photographer manually establish focus, along the same principle as the old zoom in and focus advice

  • Focal Lengths - Short to Long

    10mm 20mm 40mm

    80mm

    85mm

    150mm 300mm

  • Field of View Lenses of different focal lengths take in more or less of a

    given scene A normal or standard lens generates images that

    generally look "natural" to a human observer under normal viewing conditions

    Lenses with focal lengths shorter than a standard lens are referred to as wide-angle, and focal lengths longer than this are referred to as telephoto

    In practice, a standard lens has a focal length approximately equal to the diagonal measurement of the sensor - for 35mm film, about 43mm, for most digital SLRs, about 30mm

  • Different Perspectives Wide angle lenses give a perspective that tends to exaggerate foreground details,

    and take in a broad swath of background. Close-up portraits taken with a wide-angle lens tend to distort features and give results that are unflattering

    Moderate telephoto lenses with focal lengths in the 80 - 135mm range (for a 35mm camera) are considered classic portrait lenses because they give a pleasing perspective - they also require the photographer to stand well back from the subject, well out of their personal space

    Telephoto lenses tend to compress distance and because they only take in a small slice of the background, can make the background seem closer than it is. Wide-angle lenses by comparison emphasize the distance between foreground and background

    The longest (non-military) telephoto lens is the Zeiss Apo Sonnar T* 4/1700, a 1700mm f/4 lens custom-built at a cost of $200,000. It comes with a 2x telephoto extender, resulting in a 3400mm f/8 lens. The front element of the lens is 17 across. It fits on a Hasselblad 6x6 medium format camera, and weighs 564lb.

  • Primes and ZoomsPrime lenses Zoom lenses

    - Fixed focal length + Variable focal length

    + Best image quality - Good image quality

    - Change view = Change lenses + Vary your view with a twist

    + Large maximum aperture - Small, often variable maximum aperture+ Lower cost - maybe - More expensive - maybe

    + Light weight, simple - Heavy and complex

    Highest-quality image Convenience & flexibility

  • Cin Lenses vs. Still Lenses Recently, several manufacturers (eg Zeiss) have announced adapters so you can

    mount certain cin (or PL) lenses on still cameras This is primarily intended for use with DSLRs in video mode Whats the big deal? Still-camera lenses generally have three properties that make them unsuitable

    for cin usage: Not parfocal - this means that focal distance changes as you vary the zoom Ramping - this means that aperture changes as you vary the zoom Breathing - this means that focal length changes as you adjust focal distance

    In addition, cin lenses are generally of very, very high quality optical design & manufacture to minimize flare and vignetting, and to maximize contrast and acuity. They may also have externalized and/or remote zoom & focus controls

    Even (eg) Canons L-series lenses are considered entry-level in the cin world

  • Optical vs. Digital Zoom Digital zoom is really just cropping and enlarging pixels & reduces image quality

    Optical zoom gives better image quality

    Optical Digital

  • Apertures - Light Control Many lens systems employ a

    diaphragm that creates a variable-size aperture or to vary the amount of light that is allowed through the lens. This also controls depth of field

    In camera lenses, this is usually referred to as an aperture stop, and the usual design is an iris diaphragm

    In human eyes, it is called the iris. The pupil is name for the aperture or hole in the middle.

  • Image Stabilization (IS) Also known as Vibration Reduction (VR), Anti-Shake (AS), Optical Stabilization (OS)

    etc Reduces blurriness in photos by reducing the effect of camera shake - NOT

    movement of the subject Good systems give 3-4 stops of vibration reduction - eg you can use a shutter speed 8

    or 16 times slower than the (1/focal length) that is recommended to avoid camera shake

    Some IS systems have a pan mode where they will ignore lateral movement - IS can also go a little haywire when the camera is very steady, eg on a tripod and should be turned off in these circumstances if necessary

    ISO 100, f/5.6, 1/5 s, 85mm focal

    length

    IS off IS on

  • Optical vs. Digital IS As with zoom, analog/optical is better than digital Digital IS results in lower resolution pictures, just as

    digital zoom does Analog/optical systems compensate for camera

    movement with counter movements (movement is detected by gyroscopes or MEMS accelerometer)

    moving lens/prism system inside the lens barrel - optimized for each lens

    moving sensor (eg Sony !-Series) - buy it once, makes any lens vibration resistant on that body, within limits

  • f Numbers, Aperture & Focus

    f numbers can be a little confusing ... The smaller the number, the larger the

    aperture, the more light is let in, and the shallower the depth of field

    f numbers are a ratio of focal length to aperture size - so when the focal length stays the same, and the aperture gets smaller, the f number goes up

  • f Numbers in Practice The laws of physics limit the largest possible aperture to about

    f/0.5 - Stanley Kubricks Barry Lyndon was shot with an amazing NASA-developed f/0.7 lens that allowed the director to shoot the interior scenes by candlelight

    Lenses with a large maximum aperture ( = small f number) tend to be very large in diameter, and therefore expensive

    The iris of the human eye opens up to a maximum relative aperture of about f/3.2 in dim light, and closes down to a minimum of about f/13 in bright sunlight

    By convention, the typical step between f numbers is the square root of 2 - roughly 1.4x. Each step between f numbers represents a doubling or halving of the amount of light. The actual figures are typically rounded up or down for convenience.

    f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32 ...

  • Aperture & Depth of Field The depth of field (DOF) is the

    portion of a scene that appears sharp in the image.

    Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on either side of the focused distance, so that within the DOF - or zone of acceptable sharpness - the image appears sharp

    A wide aperture (small aperture number) results in a shallow depth of field, a small aperture gives more depth of field

    Flowers at f/5.6

    Flowers at f/32

  • Aperture & Depth of FieldEffect of aperture on blur and DOF. The points in focus(2) project points onto the image plane(5), but points at different distances(1 and 3) project blurred images, or circles of confusion. Decreasing the aperture size(4) reduces the size of the blur circles for points not in the focused plane, so that the blurring is imperceptible, and all points are within the DOF.

  • The Hocus-Pocus of Focus Focus is subtle, and not an absolute concept - only things that are

    acceptably sharp Acceptable sharpness depends upon the resolving power of the

    human eye and the display medium Example: image that looks sharp in small preview on the LCD but

    when you blow it up on the computer monitor it looks soft Focus is related to the circle of confusion - an optical spot caused by a

    cone of light rays from a lens not coming to a perfect focus when imaging a point source (the smallest imaginable point of light - in practice, think of a distant star)

    When the circle of confusion is smaller than the resolving power of the eye, sensor pixel, or film grain, an image will appear to be in focus

    Conversely, when sensor pixel size is smaller than the smallest circle of confusion a lens system can produce, then smaller pixels will not result in more usable resolution - megapixel madness

  • Ball of ConfusionThe depth of field (or zone of acceptable sharpness) is the region where the size of the circle of confusion is less than the resolution of the human eye (or of the display medium). Circles with a diameter less than the circle of confusion will appear to be in focus

  • Depth of Field and Distance For a given lens, depth of field varies with focal length, aperture,

    and focal distance Shallow depth of field

    Long focal length (telephoto) Wide aperture (small f/number) Close focus (focused on near subjects)

    Deep depth of field Short focal length (wide-angle lens) Small aperture (large f/number) Far focus (focused on distant subjects)

    See http://dofmaster.com for example calculations

  • Hyperfocal Distance Two closely related definitions giving almost identical results:

    1. The closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp; that is, the focus distance with the maximum depth of field. When the lens is focused at this distance, all objects at distances from half of the hyperfocal distance out to infinity will be acceptably sharp.

    2. The distance beyond which all objects are acceptably sharp, for a lens focused at infinity.

    Varies with focal length, f number (aperture) and the size of the circle of confusion

    See http://dofmaster.com for example calculations

  • Diffraction-Limited Optics Up to a certain point, using a smaller aperture will result in

    a bigger DOF = more sharpness in the image Beyond a certain point though, diffraction effects will limit

    image resolution With a very small aperture, the image is formed only

    through the center portion of the lens glass, magnifying the effect of defects in the material & manufacturing

    A lens image-forming powers can be limited by many factors, but the ultimate physical limit is due to diffraction

    Thus, good lenses are said to be diffraction-limited - meaning their performance is limited only by the laws of physics

  • Lens Hoods Helps to shield your lens from stray, off-axis

    light rays Can help to reduce flare and increase image

    contrast - allegedly .. May have greatest value in helping to

    physically protect the front surface of your lens Tend to be expensive for what they are, may

    help improve image quality, and can definitely help to physically protect the lens

  • Closer & Closer A large part of photographing small things at high magnification involves

    getting closer to the subject, so this type of photography is often known as close-up

    Macro or close-up mode on compact cameras may unlock/unlimit some part of the lens system to allow it to focus closer at the expense of distance focusing - not magic

    Close-up focus with a DSLR can be achieved in one of two ways: With lenses - including adapters, screw-in filters or dedicated macro

    lenses, reversing rings for primes, that all optically augment or replace an existing lens

    Without lenses - eg bellows or extension tubes that physically move the lens assembly further from the film plane

    With a zoom, greatest magnification is typically found at the long end of the zoom range, even though it also increases minimum focus distance - so even though you cant get as physically close to the subject, the lens greater magnification more than makes up for it

  • A Closer Look at Macro Historically, macro refers to a camera system with the

    ability to reproduce an object at 1:1 (the magnification ratio) or greater on the film/sensor

    Lens specifications often include a maximum magnification ratio - everyday lenses typically around 0.1x or 1:10 - ie image on sensor is 1/10 size of real object

    Specifically-designed macro lenses often give at least 1:1 magnification; when combined with bellows or extension tubes, magnifications of 10:1 or higher may be achieved

    At the extreme, starts to overlap with microphotography - photography with a microscope

  • Small Subjects, Big Challenges At macro scales, DOF becomes extremely shallow, demanding very

    small apertures, and consequently lots of light, ie bright sunlight or flash - which also helps to minimize the effect of vibrations through very short exposure times

    Camera-mounted flash is often blocked by body of the the lens itself - hence the invention of the macro ring flash

    Can only stop a lens down so far before diffraction effectsdominate and quality suffers

    Even at smallest aperture, DOF may become so shallow that only part of the subject is in focus - may alleviate with focus stacking software eg CombineZP or Helicon