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8/6/2019 Dslr Notes 1

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The Exposure Triangleunderstanding camera basics and functions

By Alistair Keddie

To photographquite literally means to draw with light and by understanding

how light enters our camera and subsequently, how we affect and control

that, then we open up to a new world of photography. Taking a photograph

is technically speaking, actually the act of making an exposure. Its called anexposure because we physically expose a light sensitive material, the

cameras digital sensor, to light.

Generally we control the exposure by understanding the combined affect of

three main factors and how, individually these affect the visual statement we

want to make. These factors are, ISO which controls how sensitive ourcamera is to light, Shutter which controls the length of time our sensor if

exposed to light, and Aperture, which controls how much light enters our

camera.

Changing any one of these affects the other two and is why we representthem together as the Exposure Triangle.

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Understanding ISO

ISO is a measure of how sensitive the camera is to light. The higher the ISO

the higher the sensitivity orthe faster its speed and the less lightwell need to

make an exposure. For instance, as day falls into evening, we increase our ISO

to a higher or faster setting increasing our cameras sensitivity to the lower

light levels.

The lower the ISO, the lower thesensitivity or the slower its speed and the

more lightwe need to make an exposure. For instance, on a sunny or brightday we can use a low orslow ISO like 100 or 200.

This begs the question, why not set the camera to a fast ISO to cope with all

lighting conditions?

We dont have a single ISO setting because the higher or faster we make thesensitivity, the more noise will be introduced into our exposures which can

degrade our image quality. This noise across the image can get in the way

and lead to a loss of fine detail. Colour also suffers at higher ISOs and begins

to desaturate and lose strength. Likewise, high ISOs tend to produce more

contrasty images, losing finer tonal gradients and ranges.

We therefore aim to use as low an ISO as conditions will allow. This producesfiner image quality with virtually no noise or loss of fine detail as well as

capturing strong, saturated colour and finer tonal ranges and gradients.

This is not to say that using high ISOs will result in poor photographs, quite the

contrary. But it is necessary to be aware of the visual impact of raising ISO.

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For instance, in the days of high speed film (high ISO) these tended to be

black and white producing high contrast grainy images with a very distinctive

look or aesthetic to rock and roll, documentary or even war photography

where, working hand held without flash in low light you had no alternative.

Film grain however is very different to digital noise and is often very beautiful

in its own right lending considerably to the visual look and feel of the

photography. Digital noise though is a different matter. Untreated, it ugly

and can look like a green and red porridge covering our shot. The camera

does usually have some built in noise reduction and we can do more on the

computer to make noise more attractive, more part of our image.

Typical ISO settings for general use.

100 200 ISO:

Generally good for daylight out of doors hand held capture giving very low

noise and fine image detail.

400 800 ISO:

Use as the light begins to fall or indoors without flash. Usually still gives good

detail but more significant noise added as the ISO rises.

1600 ISO and above:

Use in low light conditions where you have no alternative. Significant noise will

be added with loss of fine detail at these ISOs. Provided you can work with

this, there is no reason not to use them.

ISO directly affectsshutter speed. The higher the ISO, the higherthe cameras

sensitivity to lightand the faster the possible shutter speeds we can expect.The lower the ISO, the lower the cameras sensitivity to lightand the slowerthe shutter speeds become. This can be an issue if the shutter become too

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slow for hand held capture and our images start to blur. On the plus side,

raising ISO can allow us to safely shoot hand held in low light.

As well discover later when we understand the relationship between depth of

field and aperture, well realise that to get the best out of a landscape shot

wed use a low ISO to increase the fine detail and colour saturation. This

combined with a small aperture leads to slow shutter speeds and the needfor a tripod.

Understanding the Shutter

We usetheshutterto control the exposure by varying the amount of time theit is open for. This can range from very fast to very slow. Shutter speed areexpressed in fractions of a second, and usually range from 1/4000 of a second

all thye way down to 30 seconds.

Your camera will normally identify the shutter speed as a fraction on its display

or may only use the lower number, for instance 30, 100, 500 etc to indicate

1/30, 1/100 and 1/500.

Forslower shutter speeds, the display changes, usually after 1/4 sec to show

03 to indicate 0.3 seconds, then 04, 05, 06, 08, 1 etc. 1 is one second

with the quotation mark indicating seconds. From there it drops all the way to

30. Much lower shutter speeds are also possible but require a remote cable

release to operate properly with the camera set to Bulb mode.

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We think about shutter speed in relation to motion and the way we want to

capture it. For instance, to freeze motion we would use fastshutter speeds to

capture a water splash or an insect in flight. We can also use slow shutter

speeds to creatively blur motion such as smoothing out the motion of water

as it tumbles over a fall or of traffic at night as it moves along, the headlights

turning into a river of light.

We play with shutter speed to vary how sharp or how much blur we want to

introduce into a subject in motion.

A general rule of thumb around shutter speed is

when they fall to around 1/60 sec you need to

start thinking about better supporting or using a

tripod. Some cameras have built in ImageStabilisation, usually indicated by the letters IS or IOS. With this turned on, it ispossible to shoot hand held at even slower speeds. Bear in mind that the

slower the shutter speed, the more you need to support the camera to

reduceshake. Camera shake is more of an issue when using a longer lens tozoom as any shake will also be magnified. Using longer lenses generallymeans working with faster shutter speeds. The general rule here is worry whenthe speed begins to fall below the focal length of the lens. For instance,shooting at 200mm would mean trying to use speeds of 1/200 and above.

Shutter speeds affect both ISO and Aperture. In order to achieve faster shutterspeeds you might need to increase ISO which introduces more noise, or you

might want to open the aperture, which decreases the depth of field.

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Understanding the Aperture

We use Aperture to control exposure by adjusting the amount of light that

enters the camera. We do this, by varying the size of a hole or diaphramwhich we can open or close. This is the aperture and is generally housedinsidethelens.

The size we set this to is called the aperture value (Av) and is measured in F

Stops where the f stands forFocal Ratio. Confusingly, the smaller the f stop,the bigger the aperture whereas, the bigger the f stop, the smaller the

aperture.

The F Stop Scale measures how much light enters the camera and each full

stop down lets in half as much lightas before. Indeed, the aperture is half the

size of the stop before it. The values used to describe this scale are F

numbers. These are worked out by multiplying by the square root of 2 and

give a sequences as follows...

F1 one full stop down is = 1/2 the light at F1 F1.4

F1.4 one full stop down is = 1/4 the light at F1 F2





F8 one full stop down is = 1/128 the light at F1 F11



F22 one full stop down is = 1/1024 the light at F1 and so on

Because it is a mechanical device, the aperture can in theory be closed

down by any amount however as well as the full stop scale above it is often

also measured in thirds of a stop. That is, for instance between F2.8 and F4

well find F3.2 and F3.5 or between F5.6 and F8 the values F6.3 and F7.1.

These are fractional f stops.

By controlling the size of the aperture we control the amount of light entering

the camera in measured increments and depending on its size, whetheropen

or closed will have a profound affect on ourDepth of Field (DoF).

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UnderstandingDepth of Field

Depth of fieldcan be a vital creative tool in our photography which we use,

for instance, to blur and soften the background behind a portrait or capture

as much detail from foreground to horizon in a landscape as possible. It is a

term which describes the amount of our scene that is in acceptable focus.

Not what the camera is focussed on, the point of focus but rather how that

point comes into focus as we move towards or away from it. It is in fact howfocus behaves over distance.

If for instance, our camera is focussed on our subject, say a person, the

autofocus measures the distance to the subject and sets the lens sharp at

that distance. Our person comes out sharp in the photograph. If I introduce

another person standing much closer to the camera, they come out blurred.

They only become perfectly sharp when they are standing at the same

distance from the camera as our original subject. So focus is shifting across

distance to thepointwe have focussed on. As this transition occurs, there isan area in front of, or behind our subject where the scene will be acceptably

sharp and that is the depth of field. Using the aperture we can control how

wide orshallow this depth of field will appear. That is, choose whether to

capture our background soft and out of focus, perfect for a portrait, or sharp

and well defined, ideal for a landscape.

Controlling Depth of Field

We use the aperture to control depth of field. Opening the aperture will

create progressivelyshallowerDoFs whilst closing it will widen ordeepen the

DoF. That is, the lower the F Stop of the aperture, the shallower the depth of

field and the higher the F Stop, the wider the depth of field.

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An example of how depth of field shifts as the aperture value changes. Wide

open apertures create much shallower dofs whilst more of our scene

becomes acceptably sharp as we close the aperture down.

In the next section, part 2, well look at introducing the DSLR Camera.

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