a forensic scientist's guide to photography brian j

A FORENSIC SCIENTIST’SGUIDE TO PHOTOGRAPHY

– DO NOT CROSS CRIME SCENE – DO NOT CROSS CRIME S

Brian J. Gestring

Copyright © 2007 by Brian J. Gestring, Bridgewater, NJ 08807. All rights reserved. This publication was specifically designed and formatted for free use as a required reading for the American Board of Criminalistics Certifications Examinations. Use and or distribution for other applications are prohibited without expressed written permission from the author. The author can be reached through the “CONTACT US” tab on the American Board of Criminalistics website(www.criminalistics.com).

Contents

Introduction 1 Underlying Theory 2 The Camera 4 Exposure 14 Lighting Techniques 17 Filters 20 Crime Scene & Evidentiary Photography 23 Overview of Digital Photography 28 Choosing a Photo System 32 Authors Note 33 Acknowledgements 33 Bibliography 34

A Forensic Scientist’s Guide to Photography/ Gestring

1

INTRODUCTION

Since its inception nearly 200 years ago, photography has been integrated into nearly every

aspect of our lives. Everywhere we turn we are bombarded with visual imagery of one form or

another. We are also regular contributors to this montage. With the advent of new cellular phone

technology, more people than ever now carry cameras. Yet even with this abundance of cameras

few people actually know how a camera works.

The mystery seems to be further intensified by differences inherent in photography’s diverse

application. The photographer must know in advance what they are trying to depict in order to

successfully document it. For instance, photographing a footwear impression is completely different

than photographing a model. Models may be photographed with shallow depth of field to keep the

viewers attention focused on the subject and with diffuse illumination to hide any skin blemishes or

imperfections not hidden by make-up. Conversely, a footwear impression will be documented with

significant depth of field, so everything is in focus, and oblique illumination to highlight details

caused by the three dimensional nature of the impression.

While newer cameras often have point-and-shoot settings which make them more appealing

to the masses, an understanding of basic photographic fundamentals is needed to take good pictures.

Written specifically for practicing forensic scientists, this guide will take readers through the basics

of photography with specific emphasis on forensic applications. Starting with conventional film

cameras, it will progress into digital photography and then discuss how to choose a camera system to

meet your needs.

Reading for the American Board of Criminalistics Certification Examinations

2

2

UNDERLYING THEORY

Literally translated from its Greek origins, photography means writing with light. So it

makes sense that to understand photography you must understand the nature of light. Light can

propagate as either a particle or a wave, but is characterized by its wave length. Visualizing a typical

sin wave, the wave length would be the distance between two peaks and is measured in nanometers

(1 nm = 10-9 meters). Light sources like the sun emit various wavelengths of light across the

electromagnetic spectrum.

Figure 1: Electromagnetic spectrum. (Courtesy of Stacy Ann Collins, Pace University)

We are most familiar with an area of the electromagnetic spectrum called the visible

spectrum which contains energy that we can see with the unaided eye (between 400 and 700 nm),

but photography is not limited to the visible spectrum. Forensic photographers often take

advantage of both ultraviolet and infrared to document various forms of evidence. Most black and

white panchromatic films are sensitive to energies from around 320 nm to around 1350 nm. Clearly

better results are obtained from films specifically designed to record certain energies. Infrared film

is a good example of this. Traditional film has an opaque dye that prevents light from passing

through the film and bouncing off the camera back causing a glow around bright objects. This dye

is known as anti-halation layer. This layer also blocks a good amount of infrared. While infrared can

be documented with high speed panchromatic films, films that are manufactured without halation

layers and produce better results.

Understanding how to gauge both the quality and quantity of available light will ensure that

the resulting picture will be a good exposure and a true and accurate depiction of what was

photographed. Another way to refer to the quality of light is to refer to the color temperature.

Color temperature is a theoretical way to evaluate the degree of whiteness of a light source. The

concept of color temperature is based upon a theoretical black body. The black body does not emit

any light. As heat is applied to the black body, it begins to emit light. The temperature that is

applied to the black body corresponds to the degree of whiteness of the light that is emitted and is

measured in degrees Kelvin (1 degree Kelvin=-273 degrees Celsius). Since commercially available


3

films are balanced based upon the color temperature of the light source, it is important to take this

into consideration when recording an image. Most films are balanced toward daylight (around 6,000

degrees Kelvin). If photographs are taken outside or inside with the camera’s flash (which also has a

color temperature of around 6,000 degrees Kelvin), the colors recorded will be a true and accurate

depiction of what you saw. If the color temperature of the film is different than that of the light

source, there will be a color shift. An example of this would be photographing someone blowing

out the candles on their birthday cake in a dark room with no flash. Candles are approximately

1,800 degrees Kelvin. Recording the image with daylight balanced film will result in an orange color

shift.

Figure 2: Photograph of candles being blow out without flash (left), and with flash (right) with daylight balanced film.

Another example would be if you take a picture with daylight balanced film and no flash under

fluorescent light (around 4,200 degrees Kelvin). The resulting color shift will make the picture

appear greenish.

Figure 3: Photograph taken under fluorescent light (left) and with flash (right) with daylight balanced film.


4

4

The quantity of light is based upon the intensity of the light source and the distance of the

object from the light source. Common sense would dictate that as the distance from the light source

to the object increases, the amount of light striking the object will decrease. The inverse square law

predicts that the intensity of a point light source will decrease in an inverse proportion to the square

of the distance. In reality this means that if the distance from the light source to the object is

changed from 1 inch to 2 inches, the inverse square law would predict ¼ as much light.

THE CAMERA

In crude terms, a camera is simply a box capable of limiting the amount of light that enters

inside of it. Inside of that box is film which is a media sensitive to light. This means that the amount

of light required to create an image is regulated by three essential parameters: the sensitivity of the

film to light, the length of time the camera shutter is left open, and the diameter of the lens opening.

Since adjusting any one of these variables affects the other two, they must be related in some

manner. In photography that relationship occurs through calibrated increments known as a “stop.”

An adjustment of a full stop will either halve or double the amount of light.

Both modern digital cameras and conventional film cameras refer to the sensitivity of the

recording medium as the film speed or exposure index. Traditional film is a photosensitive material

held in a gelatin like emulsion. The photosensitive materials are silver halide crystals. These crystals

will react proportionally to the amount of light that strikes them. The size of the crystals will

determine the film sensitivity. Films with larger silver halide crystals will be more sensitive because

it takes less light to create an image, but would have lower resolution because less silver halide

crystals are recording the image. Conversely smaller silver halide crystals will need more light to

create an image, but have higher resolution.

Figure 4: Representation of large silver halide crystals in fast films (left) and small silver halide crystals in slow films (right).

In the United States film speed was rated by the American Standards Association and was

commonly referred to as an ASA. A low ASA meant the film had small silver halide crystals. As the

size of the crystals increase, so would the ASA. In Europe, film speed was rated on a logarithmic

scale known as the German Industry Standard or Deutsche Industrie Norman (DIN). Just like the


5

ASA, the DIN values would increase with the film’s sensitivity to light. The two methods were

eventually merged into one by the International Organization for Standardization (ISO). The ISO

lists the ASA value then the DIN number in degrees after a back slash. ISO and ASA have become

almost synonymous.

Figure 5: Photo of an ISO rating on a film canister.

Figure 6: Table comparing ASA and DIN values.(Courtesy of Stacy Ann Collins, Pace University)

The value displayed on the camera which is used to calculate exposure tends to be the ASA.

Stops ASA Stops ASAFull 6 2/3 320 1/3 8 Full 400 2/3 10 1/3 500 Full 12 2/3 640 1/3 16 Full 800 2/3 20 1/3 1000 Full 25 2/3 1250 1/3 32 Full 1600 2/3 40 1/3 2000 Full 50 2/3 2500 1/3 64 Full 3200 2/3 80 1/3 4000 Full 100 2/3 5000 1/3 125 Full 6400 2/3 160 1/3 8000 Full 200 2/3 100001/3 250 Full 12500

Figure 7: Table of film speeds.


6

6

The mechanism that regulates how long light is exposed is a mechanical device known as the

shutter. There are two fundamental shutter designs. A leaf shutter is housed in the camera lens and

is very quiet.

Figure 8: Diagram of leaf shutter operation. (Courtesy of Stacy Ann Collins, Pace University)

Focal plane shutters are housed in the camera body and are, by far, the most common.

Instead of being a solid wall that moves in front of the light sensitive elements, focal plane shutters

are made up of several overlapping elements that sweep across the film plane. They can either be

vertical or horizontal. The distinctive sound that everyone associates with a picture being taken is

the sound of a focal plane shutter in operation.

Figure 9: Representation of focal plane shutter operation.

Effective control of shutter speed will allow photographers to stop motion in the resulting

image. For the most part this motion is from the subject. Sometimes it can be introduced by the


7

photographer through camera shake. Camera shake occurs when the shutter speed is too slow for

the camera to be hand held for a given exposure.

Figure 10: Photos taken with different shutter speeds. Note the blurriness in fixed objects at the slowest speeds (upper left). This is induced by camera shake. The lower right shows how faster shutter speeds aid in stopping motion.

Shutter speeds can range from fractions of a second to hours and are arranged in stops. Changing

the shutter speed one full stop will result in either doubling or halving the amount of light.

Stops Shutter

Speed

Stops Shutter

Speed

Stops Shutter

Speed

Stops Shutter

Speed

Full 1/8000 1/3 1/800 1/2 1/90 1/3 1/6

1/3 1/6400 1/2 1/750 2/3 1/80 1/2 1/5

1/2 1/6000 2/3 1/640 Full 1/60 Full ¼

2/3 1/5000 Full 1/500 1/3 1/50 1/3 1/3

Full 1/4000 1/3 1/400 1/2 1/45 Full ½

1/3 1/3200 1/2 1/350 2/3 1/40 1/3 1/0.7

1/2 1/3000 2/3 1/320 Full 1/30 Full 1.0”(sec)

2/3 1/2500 Full 1/250 1/30 1/25 1/2 1.5”

Full 1/2000 1/3 1/200 1/2 1/20 Full 2.0”

1/3 1/1600 1/2 1/180 Full 1/15 1/2 3.0”

1/2 1/1500 2/3 1/160 1/3 1/13 Full 4.0”

2/3 1/1250 Full 1/125 1/2 1/10 1/2 6.0”

Full 1/1000 1/3 1/100 Full 1/8 Full 8.0”

Figure 11: Table of shutter speeds


8

8

The last factor in your photo system that controls the amount of light for a given exposure is

the size of the lens opening. The diaphragm that controls the size of the lens opening is known as

the aperture. The different aperture settings that each lens is capable of occur in calibrated

increments known as f –stops or stops.

Stops f # Lens Marking Stops f # Lens Markings

Full 1.000 1.0 1/3 8.98 1/3 1.122 1/2 9.514 1/2 1.189 2/3 10.079 2/3 1/260 Full 11.314 11 Full 1.414 1.4 1/3 12.699 1/3 1.587 1/2 13.454 1/2 1.682 2/3 14.254 2/3 1.782 Full 16.00 16.0 Full 2.000 2.0 1/3 17.959 1/3 2.245 1/2 19.027 1/2 2.378 2/3 20.159 2/3 2.520 Full 22.627 Full 2.828 2.8 1/3 25.398 1/3 3.175 1/2 26.909 1/2 3.364 2/3 28.509 2/3 5.564 Full 32.000 Full 4.000 4.0 1/3 35.919 1/3 4.490 1/2 38.055 1/2 4.757 2/3 40.318 2/3 5.040 Full 45.255 45 Full 5.657 5.6 1/3 50.797 1/3 6.350 1/2 53.817 1/2 6.727 2/3 57.018 2/3 7.127 Full 64.000 64 Full 8.000 8.0

Figure 12: Table of f -stops

In photography, the size of the lens opening is directly related to the depth of field depicted

in the resulting image. While a large lens opening allows more light, it provides a shallow depth of

field. Squinting can illustrate how depth of field increases as the diameter of the aperture decreases.

Figure 13: Illustration of how squinting increases depth of field. (Courtesy of Stacy Ann Collins, Pace University)


9

The eye is naturally focusing on a distance that is either nearer or further away than the object.

Reducing the aperture allows everything from the point where the eye can see to the object to come

into focus.

Figure 14: Photographs taken with a 60 mm lens and different lens aperture settings. Note the increase in depth of field as the f # increases.

These basic principals of film speed, shutter speed, and aperture apply to all photo systems

and camera formats. In general, cameras are defined by their format. Format in this context is

referring to the size of the negative that the camera is using to record the image. For example a 35

mm camera uses film that is 35 mm in width. While the width from edge to edge is 35 mm, the

actually area where an image is recorded is much smaller.

Figure 15: Diagram of 35 mm film negative


10

10

Over the years, there have been many film formats. The most popular formats are 35 mm,

medium format, large format, and now digital. Medium and large format have negatives that are

much larger than 35 mm. A negative for medium format can either be 2 ¼” x 2 7/8” or 2 ¼” x 1

5/8”. For a large format camera the negatives can either be 4” x 5” or 8” x 10”. The size of the

image sensor used to capture the image in digital media will vary with manufacturer.

Larger negatives yield better resolution images. To illustrate this point, imagine

photographing a footwear impression with a 35 mm camera. If the footwear impression is 12 inches

in length and the picture is taken so that the footwear impression fills the entire camera view, the

actual image recorded cannot be larger than 36 mm in length which is the length of the negative.

This type of evidence is often enlarged to life size for analysis. In order to enlarge that image to life

size, the negative would have to be enlarged almost 8 ½ times the original size. Minimizing the

enlargement needed increases the detail that can be visualized in the resulting image.

Figure 16: Illustration of an image in real life and on a 35 mm negative. (Courtesy of Stacy Ann Collins, Pace University)

The relationship between the size of an item in real life and the size of the object in the

resulting print is referred to as “reproduction ratio.” As an example, if an item is photographed with

a scale, the scale can be used to determine the reproduction ratio of the resulting image. If 2 cm on

the photographed scale correlates to 1 cm on a real scale the reproduction ratio is (1:2) which is

double the original size.

Like most things in photography, one size does not fit all. The pros and cons for each

format need to be evaluated for each application. The 35 mm format has the smallest negative

format and can be divided into either point-and-shoot cameras or single-lens reflex (SLR) systems.

The point-and-shoots tend to have built-in camera lenses that are not interchangeable, a separate

optical system for focusing and taking the picture, and built-in flash units. By contrast, SLRs allow


11

different lenses to be placed on the camera body, flash units to be used on or off of the camera, and

the photographer to view the picture through the lens that will be recording the image. Most SLRs

will also have a light meter built into the camera body.

Figure 17: Point and shoot camera (left), and a single lens reflex camera (right).

Despite the small size of the negative, 35 mm has immerged as the most versatile camera

format. Comparatively, SLRs are relatively inexpensive and allow photographers an incredible

selection in camera lenses, flash units and other accessories. Medium format cameras are at least

twice as expensive as 35 mm’s and have some other quirks. Just as with the 35 mm point-and-

shoots, it is not uncommon for medium format cameras to have a separate lens for viewing and

recording the image. This can become problematic when performing close-up photos. In these

situations it is possible that the camera lens will be seeing something different than the viewing lens.

This phenomenon is known as parallax error. Medium format cameras also might not have built-in

light meters. On the positive side, the film is loaded into the camera in a way that allows

photographers to easily change film between each shot. It is also possible to get Polaroid film and

digital adapters. These adapters allow professional photographers to preview lighting and exposure

conditions prior to committing an image to film.

Large format cameras are considerably more expensive than 35 mm’s and share some of the

same pros and cons of medium format. The increase in resolution must be weighed against the cost

and versatility needed for each application.

Figure 18: Medium format camera (Left), and a large format camera (right). (Photos courtesy Keith Freitag)

Another factor that affects the quality of the image is the camera lens. Camera lenses are

defined by their focal length. The term focal length refers to the distance from the optical elements


12

12

in the camera lens to the film plane. Like any lens, camera lenses are subject to distortion. Normal

lenses have the least distortion. The view represented by a normal lens is considered the normal

perspective and is what we would see with one eye closed. To duplicate the normal perspective a

lens with a focal length similar to the diagonal of the film plane is needed. If we consider 35 mm

format, the diagonal of the film plane is about 43 mm.

Figure 19: Illustration of 35 mm film diagonal.

Photographers generally regard 50 mms as a normal lens for a 35 mm camera. Focal length

less than 50 mm provides a broader view of the subject and are considered wide angle. Focal

lengths of greater than 50 mm provide a tighter view of the subject and are considered telephoto.

Since the normal perspective is based on the diagonal of the negative, if the size of the negative

changes, then the focal length of the normal lens must first be determined from the diagonal of the

film plane before the other relationships can be determined.

Since any deviation from normal introduces a level of distortion. It is important to

understand how the choice of focal length affects the resulting image. To illustrate this point, if a

rectilinear grid is photographed with a normal lens it will appear as a grid. If that same grid is

photographed with a wide angle lens, the magnification will decrease from the center of the image

producing barrel distortion. If the original grid is photographed with a telephoto lens, the

magnification will increase from the center of the image manifesting itself as pincushion distortion.

Figure 20: A rectilinear grid (left), the same grid exhibiting barrel distortion (center), and pin cushion distortion (right). (Courtesy of Stacy Ann Collins, Pace University)

Initially, optical technology only allowed for fixed focal length lenses. Photographers were

always struggling with which lenses they should carry with them. Within the last quarter century

significant technological advances have made quality variable focal length lenses a reality. These

“zoom” lenses allow a single lens to cover from wide angle to telephoto.


13

While the focal length of a lens provides an indication of the view provided by the lens, it

also provides a valuable rule of thumb regarding shutter speed selection. If existing light is being

used to capture an image, the shutter speed selected must always be greater

thanLengthFocalLense

1 . This rule ensures that blur is not introduced into the image through

camera shake.

Within each lens is also a mechanical device that can control the physical size of the lens

opening. This opening or aperture is responsible for controlling the amount of light that is allowed

through. Lenses that are capable of large openings are considered “fast lenses” because they allow

images to be formed under low light conditions. The down side to wide open apertures is that they

result in images with little depth of field. As the aperture size decreases, depth of field increases.

While there are reasons for desiring shallow depth of field, for the most part, the majority of

forensic applications require significant depth of field.

The value that correlates to the size of the opening is known as the f # or f-stop and is

representing by the equation:

OpeningLenstheofDiameterLenstheofLengthFocalf

# =

This equation demonstrates that the f-stop is inversely proportional to the diameter of the lens

opening and is related to the focal length. It is important to keep both of these factors in mind when

selecting the desired aperture (f #). The lens with the smallest opening will provide the best depth of

field. Since focal length is incorporated into the calculation, this does not always mean the lens with

the lowest f #.

Figure 21: Two images recorded with the same f # with different focal lengths. The image on the left was taken with a 19 mm focal length. The image on the right was taken with a 200 mm focal length. Note the difference in depth of field in the images due to the diameter of the lens opening.


14

14

Everything in photography involves give and take. While higher f #’s provide greater depth

of field, the smaller lens openings also greatly reduces the amount of light requiring longer

exposures or faster films.

Understanding how lens aperture and shutter speed affect the resulting image is also

important for selecting the mode you would like to shoot in. Most cameras allow many different

levels of operation. Even the most expensive SLRs have program modes that essentially convert the

camera to a fancy point-and-shoot. They also allow photographers to select the parameter that is

most important for them and allow the camera to calculate the others. For example, stopping action

is critical in sports photography so the camera will have a shutter speed priority setting. Depth of

field can also be critical in landscape photography so the cameras will have an aperture priority

setting. Different cameras will also have additional program settings that allow photographers to

select the level of interaction they would like with their subjects. For the most part, forensic

photographers will use manual mode providing the user control of the resulting image.

EXPOSURE

Exposure is governed by the concept of reciprocity which is defined by the equation:

Exposure (E) = Intensity of the illumination (I) x Time of exposure (T)

This relationship allows photographers to change film speed, shutter speed, and f #, and still

maintain an equivalent exposure. At extremely short or extremely long exposures the reciprocity

relationship does not hold true. This is known as reciprocity failure and results in an unusable

exposure.

Since the lighting will vary for each situation, a device called a light meter is needed to

determine the proper exposure. Light meters can either be incident light meters, which measure the

intensity of light that strikes them, or reflected light meters, which measure the intensity of light that

reflects off of an object. Cameras that have built-in light meters use the latter. The meter will be

located in the camera body and obtain information through the camera’s lens. This type of metering

is known as through-the-lens (TTL) light metering. Incident light metering is more accurate because

it is measuring the actual light in a given situation. Reflected light metering can be fooled by the

object the light is reflecting off of. Reflected light meters determine exposure based upon an

industry standard of 18% gray. To get the best exposure with a TTL light meter, the camera should

be set to manual mode and the proper exposure should be determined by first metering off of an

18% gray card. The gray card should be placed in front of the object being photographed under the

same lighting conditions. Once the exposure has been determined, the gray card should be moved,


15

and the picture should be taken. A gray card can also be left somewhere in the picture to allow

exposure to be corrected during the printing process.

If an exposure is determined and a condition is changed, the concept of reciprocity allows

the photographer to determine an equivalent exposure. For example, an exposure is determined to

be f 5.6 and 1/30 for a handheld shot using a 60 mm lens with ISO 200 and existing lighting. The

shot will be blurry from camera shake unless: the camera lens is changed, the photographer uses a

flash or a tripod, or the shutter speed is increased to at least 1/60. The last alternative is the easiest

and can be accomplished by simply changing the f #. Referring to figure 11, changing the shutter

speed from 1/30 to 1/60 is the same as reducing the light by 1 full stop because the amount of time

the shutter is open is reduced by half. To get equivalent exposure, the f# would have to be adjusted

to provide 1 full stop more light. Referring to figure 12, changing the f# from 5.6 to 4 would be 1

full stop and from 5.6 to 8 would also be 1 full stop. Since the f# is inversely proportional to the

size of the lens opening, f 4 would increase the amount of light and provide the equivalent exposure.

In another example an exposure is determined to be f 4 and 1/200 with ISO 400 film. The

picture needs significant depth of field so the f # is changed to f 11. Changing the f # from 4 to 11

reduces the amount of light by 3 stops (see table 12). To obtain an equivalent exposure either the

film speed or the shutter speed need to be adjusted to provide 3 stops more light. In this case that

would mean either the film would have to be changed to 3200 speed (see table 7) or a shutter speed

of 1/30 (see table 11).

When a reflected light meter is used without a gray card, it is possible for the camera meter

to be fooled. For example if a black car was being photographed, the camera meter would compare

the black against the reference 18% gray. The meter would then perceive the car to be much darker

than 18% gray and calculate an exposure that would provide much more light. The resulting

exposure would be overexposed. Conversely if you photographed a white car, the camera meter

would perceive the white as much brighter than 18 % gray and the resulting exposure will be

underexposed.

Figure 22: Examples of underexposure (left), correct exposure (center), and overexposure (right)


16

16

Complicated lighting situations can also be problematic. If a subject is not uniformly

illuminated, it can be difficult to assess proper exposure. The classic example of this is someone

standing in front of a window in broad daylight. If the meter reading is taken off of the window, the

person in the image will just be a shadow. If the meter reads off of the person, the light streaming

through the window will provide a distracting overexposure. Modern camera manufactures have

designed different metering modes to try and overcome this. These modes all take different

approaches to sampling the subject. Some will choose a circle in the center of the field of view,

while others will take light meter readings from all over the field of view and then average them.

There are many variations of both of these approaches.

Figure 23: Different approaches to through-the-lens (TTL) light metering: a meter reading will only be taken from the small circle in the center (left), a meter reading will only be taken from the large circle in the center (center), a meter reading will be taken from each section and then will be averaged(right). (Courtesy of Stacy Ann Collins, Pace University)

As good as some of these meters are, they can not overcome our ignorance. In order to take

good pictures, a photographer needs to understand how the system they have will interpret what

they are trying to photograph and how to overcome obstacles. A technique known as bracketing is

the best way to ensure a good image. Bracketing means that each time a photographer tries to

document a complicated situation, they should take several exposures instead of just one. The first

picture should always be a nominal exposure (what the camera meter indicates). Subsequent

exposures should be taken to provide more and less light than the camera meter indicates. The

amount of light added or subtracted from the image is measured in stops.


17

Figure 24: Examples of exposure bracketing. The nominal exposure is on the top. Exposure bracketing to reduce the amount of light appears to the left. Exposure bracketing to increase the amount of light appears on the right. Note that sometimes the nominal exposure is not the best representation as in this case where the +1 is. LIGHTING TECHNIQUES

Forensic photography involves not just recording images but usually subjecting those images

to some form of analysis. Since every detail must be meticulously recorded, effective control of

illumination is critical. The most basic form of lighting used is known as direct illumination. In

direct illumination, the light source is coming from above roughly where the camera is located.

Figure 25: Illustration and example of direct illumination. (Illustration courtesy of Stacy Ann Collins, Pace University)


18

18

In cases where there are three dimensional details, oblique illumination is needed to visualize detail.

Oblique illumination refers to light coming at a grazing angle to the surface being documented.

Figure 26: Illustration and example of oblique illumination. Note the indented writing visible with this illumination that was not visible in Figure 25. (Illustration courtesy of Stacy Ann Collins, Pace University)

Sometimes it may be necessary to shine light through an object in order to visualize certain detail. In

these cases the photographer should be sure to take the substrate into account. Refraction and other

interactions between the light and the substrate need to be considered. This process is known as

transmitted illumination.

Figure 27: Illustration and example of transmitted illumination. In this case transmitted illumination is being used to demonstrate a physical match between the two torn edges of the note shown in Figure 25 and 26. (Illustration courtesy of Stacy Ann Collins, Pace University)

Large three dimensional objects are often the most challenging. The best way to document

samples like these is through the creative use of studio lighting. In studio lighting multiple sources of

illumination are placed around the object being photographed. This allows the photographer to


19

preview how the resulting image will look. Illumination can be moved until there is uniform

illumination without glare. Whenever any of these techniques are used it is important to take the

color temperature of the light source and the film into account to ensure correct color balance.

Flash units are already balanced for daylight as are most emulsions, but are much more

difficult to use than traditional light sources. Direct illumination often results in specular reflection.

Figure 28: Example of specular reflection caused by reflection of the flash.

Other off-camera uses of flash requires a good deal of experience to ensure that that the flash is in

the proper orientation and distance from the object. Photographers can take advantage of infrared

focusing devices often found on modern flash units to ensure proper position. An inexpensive

flashlight can also be taped to the top of the flash to achieve the same goal. Since the photographer

is not previewing the actual illumination that will be used to record the image, the photographer

must be very experienced to prevent over or under exposure, shadows, reflection and hot spots.

Figure 29: The flash was removed from the camera by means of a remote cable to avoid reflection from this complex subject. Unfortunately the flash was not pointed at the subject causing uneven and inadequate illumination.


20

20

Figure 30: Example of shadows caused by poor flash placement.

Another practical consideration with the use of flash is the synchronization speed. Flash

units are designed to synchronize with the camera’s shutter speed so that they will discharge when

the shutter is open. This sync speed is not standard and varies from one manufacturer to another.

When in manual mode, a photographer must be sure to set the flash to the correct sync speed or

risk not illuminating the entire frame.

FILTERS

In photography there is often a need to limit or enhance an image. Filters can be used over

the camera lens, over the illumination and sometimes even both. They allow photographers to be

more selective about the range of the electromagnetic spectrum they employ, the vibrational

direction of the illumination, the contrast in the resulting image, and to adjust the color temperature.

Figure 31: Example of some photographic filters: circular polarizing (top left), UV-haze filter (top center), and a number of black and white contrast filters. One of the most commonly used filters is a UV-Haze filter. This filter restricts ultra-violet

from reaching the camera lens providing clearer images in the visible spectrum. These filters are


21

inexpensive and can be left on the lens for normal operation thus providing an additional element of

protection for the camera lens. For the most part it is better to scratch a filter than a lens.

Polarizing filters are also commonly employed. These filters restrict the direction of vibration of the

illumination source eliminating glare.

When using black and white film, filters can also be used to adjust the contrast of the

resulting image. In order to understand this, it is important to understand how color is created and

perceived. There are three primary colors (blue, red, and green) which when equally mixed create

white light. There are also three complementary colors (magenta, yellow and cyan) which are

created by equal combinations of any two primary colors. Magenta is created by mixing blue and

red. Yellow is created by mixing red and green, and cyan is created by mixing green and blue.

Figure 32: Primary and complementary colors of light. Note that the primary colors of light are different than those of pigments (e.g. red, blue, and yellow) (Courtesy of Stacy Ann Collins, Pace University)

When we see a white piece of copy paper, the paper appears white because it does not

absorb blue, red or green. Since all three primary colors are effectively being reflected off of the

paper, our eye perceives the combination as white. When we see a green object, the object is

absorbing blue and red and is reflecting green. Similarly when we see a yellow object blue is being

absorbed and red and green are being reflected.

Figure 33: This annotated picture illustrates how we perceive color. In order to see the snow on the roof as white, it must not absorb red, green, or blue. Since the colors are not absorbed they are reflected allowing our eye to see their additive effect (white). The yellow color of the house is a


22

22

result of blue being absorbed while red and green are reflected. The red on the flag is a result of blue and green being absorbed while red being reflected. Black and white contrast control filters operate by limiting the color of the filter from

passing. If a green soda can was being photographed with a green filter, the green would appear

lighter than the other colors present in the image which would appear darker. The phenomenon of

a filter lightening colors similar to the color of the filter and darkening colors dissimilar to the filter

is known as the filter effect and can be exploited to document certain types of evidence. For

example, consider a green soda can with a faint bloody fingerprint. Using a green filter will serve the

dual purpose of lightening the soda can and darkening the fingerprint.

Figure 34: From left to right: Color picture of bloody fingerprint on a green soda can, black & white picture of the can, black & white image with a red filter (note the red in the image is lightened by the filter), and a black & white image with a green filter (note the red in the image is darkened by the filter in this case enhancing the bloody print).

Forensic photographers also use filters to restrict the range of the electromagnetic spectrum.

Many pen inks exhibit marked differences in the infrared that are not noticeable in the visible

spectrum. Infrared photography offers questioned documents examiners a non-destructive method

of evaluating documents for alterations. In addition to numerous questioned documents

applications, infrared photography is an effective method of documenting gunshot residue. On the

other end of the electromagnetic spectrum, ultraviolet photography is a great way to document soft

tissue injuries or the native fluorescence seen in many biological fluids. The former can range from

enhancing bruises on assault victims to trying to visualize a monochromatic tattoo on a decomposed

body. In order to take advantage of any of these properties, a filter is needed to limit to range of the

electromagnetic spectrum.

Most light sources emit energies from the ultraviolet clear through the infrared. Simply using

a filter over the camera lens that restricts the wavelengths, allows the photographer to document the

wavelengths allowed by the filter that are being reflected off of the sample. This technique is

referred to as either ultraviolet or infrared reflectance depending on which end of the spectrum is

being employed. If the photographer is trying to document fluorescence induced by a given

wavelength of light, then it is necessary to use a filter in front of the light source as well as a filter in


23

front of the camera lens. Documenting the native fluorescence of semen under ultraviolet is a good

example of this. Using a filter over both the light source and the camera lens is referred to as

ultraviolet or infrared luminescence again depending on which end of the spectrum.

CRIME SCENE & EVIDENTIARY PHOTOGRAPHY

Once the basic principals of photography are understood, it is important to understand how

they are implemented. Photography is just one method of documentation used to memorialize what

we see. How we use that method to document is also worthy of discussion. Dr. Peter De Forest of

John Jay College coined two phrases which aptly describe the mindsets of the documenters: passive

documentation and active documentation. In passive documentation, the investigator is not sure

what they are looking for or what is of significance. The documentation is being done to establish a

record. At a later date, information from that record can be evaluated. In active documentation, the

scientific method is being used. Investigators are collecting data, using conjecture to develop

theories, testing their theories, and documenting based upon their working hypothesis. It will

become apparent that both mindsets are needed to effectively document.

Establishing photography is a good example of passive documentation. Often one of the

greatest challenges faced by the judicial system is recreating what a scene really looked like for a

judge, jury and even the investigators. It is not uncommon for years to pass between the initial

investigation and the subsequent trials. A number of photographs must be taken with significant

overlap to establish the scene. A sufficient number of pictures should be taken to cover the entire

room including the floor and walls. The overlap allows the images to be related to each other

thereby providing an effective coverage of the entire area. If the camera to subject distance is

maintained, it is actually possible to stitch the images together to form a panoramic image.

Figure 35: Four photographs used to establish a room. Note the overlap between images.


24

24

Once the scene is established, additional photographs are needed to focus on areas of interest. Here

the photographer is guided by active documentation. A midrange photograph will frame the item in

question in greater detail than an establishing shot, but will still not be an examination quality

photograph. An examination quality photograph is also known as a close-up.

In order to take an examination quality photograph or close-up, the photographer must take

certain steps. A macro lens should be used on the camera; these are special lenses that are capable

of focusing close to the subject, and are manufactured to minimize distortion. The back of the

camera, also known as the film plane, must also be parallel to the evidence that is being documented.

Any deviation of the film plane will introduce distortion into the resulting image. It is also

imperative that the item being photographed completely fill the field of view. Since these images

will be analyzed by trained examiners, they will most likely be enlarged to life size or sometimes even

larger. Filling the entire field of view with the item ensures that the image can be enlarged at its

highest resolution. Lastly it is also critical to include a scale. Over the years, many improvised scales

have been used. Now significant improvements have been made to traditional rulers and tape

measures. If for some reason a traditional scale is not available and another item is used, it is

important to collect that item as evidence. While clearly not the best scenario, it will allow the most

accurate reproduction of images scaled against it. It is also important to verify that the scale you are

using is correct by comparing it against another known scale. The author has seen significant

differences even in the best of scales. Some agencies choose to use photocopied scales. Since

photocopiers do not always produce true copies, a photocopied scale can almost defeat the purpose

of using a scale at all. It is not just the inexpensive scales that can be problematic. The more

expensive ones have different issues. If a scale cost $15 it will be used more than once. In these

situations there needs to be a protocol in place to sanitize the scales between uses. Cleaning the

scales with 2 solutions, a10% bleach followed by 70% ethanol is a fast and easy way to ensure that

contamination is not introduced through the scale.

The scales specifically designed for forensic applications also have features built into them to

aid examiners that will be studying the images. The scales will usually have a circle with an X and Y

axis inside of it. These circles are to illustrate the camera position. If the film plane is, in fact,

parallel to the object being photographed, the circles will appear circular. If the camera is moved off

of that plane, the circles will appear more ovoid. The scales will usually have a gray area. This area

should be 18% gray allowing color correction to occur during the printing process. They usually will

also have black and white alternating patterns above the measurements. This will allow crude

measurements to be taken even if the numbers on the scale are not visible.


25

Figure 36 – Some common scales in use today. Always remember when non-traditional scales (e.g. coins or a business card) are used, they should always be collected with the evidence. There are times when a forensic photographer must utilize specialized techniques to

effectively document certain aspects of the scene. The following is a brief overview of some of the

more commonly encountered issues, which for the most part, have to do with lighting. One of the

places you would not expect to have a problem with lighting is on a bright sunny day, but when the

photographer does not control the lighting, they also cannot control the shadow. Evidence can be

hidden in unintentional dark spots in the resulting image. A technique known as fill flash is an

effective method of overcoming this problem. Even though the picture is being taken in broad

daylight, the photographer will use the flash on the camera. The flash will ensure that shadow areas

are effectively illuminated.

Figure 37: Photo taken with available light (left) and with fill flash (right). Note the increased detail visible on the front of the vehicle as well as the handgun by the driver side tire.

Night photography can also be challenging. When faced with documenting a large area at

night, the photographer has several options. The easiest is to utilize existing light photography. In


26

26

this technique, the camera is placed on a tripod with a remote shutter release cable. The camera is

set to manual exposure mode with an f # of 8 or 11 to achieve good depth of field. The ISO is

usually 400 or greater. Since the ISO and the f # are known, the built-in camera light meter will

allow the photographer to determine the appropriate shutter speed. Since even night scenes contain

a diverse range of illumination, photographer should choose the darkest feature in the image to

meter off of and bracket their exposures. Depending on the film speed and the amount of available

light, these exposures are usually relatively long. Photographers using existing light photography

should try and avoid any moving objects in their images.

Figure 38: Night photograph taken with existing light. Note the blurriness around the flags that were moving in a gentle breeze. Another option is to use a technique known as painting with light. This technique is effective even

for situations where there are large areas of shadow. Under these circumstances, the range of

available light makes getting a proper exposure very difficult. Painting with light allows the

photographer to paint in the scene one piece at a time using an off camera flash. It requires the

camera, two individuals, a piece of cardboard, a shutter remote cable, a tripod, and an external flash

unit. Essentially the technique takes multiple images on the same frame. The way this is

accomplished is to place the camera on a tripod with the camera set to manual mode. The shutter

speed will be placed on the “bulb setting” which means it will remain open as long as the shutter

release cable is depressed. The ISO will determine the f#. When using a slow to medium speed film,

set the f# to either 8 or 11. Reduce the size of the lens opening (f #) when using a faster film. One

individual will cover the lens with the piece of cardboard and open the shutter with the remote

cable. At this point the piece of cardboard is the barrier that is preventing light from getting in to

the lens. The other individual will then evaluate what areas are in shadow and position the flash. A


27

realistic example would be a car in a dimly lit parking lot. The flash could be positioned to

illuminate under the car. The camera person would then move the cardboard from in front of the

lens for just long enough for the flash to be manually triggered. They would then keep the shutter

open with the remote cable and the lens covered with the cardboard until the flash was positioned in

its next location and the procedure would be repeated. Each new aspect of the scene would be

painted onto the exposure with each new flash. The size of the scene and the level of available light

will dictate how many different flashes are needed to record the scene.

Figure 39: Example of painting with light. Through the use of multiple flashes on the same exposure the scene was captured as if it was daylight even though the only existing illumination was were two magnesium arc lamps. Also note how even shadow areas are illuminated. Documenting laser trajectories and the chemiluminescence produced by blood presumptive

tests like luminol can also challenge scene photographers. If the area where the sample is can be

darkened, the same technique can be used to document both phenomena. Sequential image burning

is a technique very similar to painting with light. Several images will be burned onto the same frame.

To document a laser trajectory, set the camera up as was done with painting with light. Turn all the

lights off in the room where the sample is. While covering the camera lens with the cardboard, open

the camera shutter with the release cable. Another individual will take a piece of white paper and

block the laser beam at the source so that only a red dot is visible. The cardboard is moved from

the lens and the path of the laser is slowly traced in the dark room. When the entire trajectory has

been traced, the lens is covered with the cardboard again. The second individual will grab the

manual external flash and position it to paint the rest of the room in. When they are ready, the

cardboard is moved again just long enough to set off the flash and then close the shutter. The

resulting image will show the trajectory clearly with all of the room details.


28

28

Figure 40: Example of sequential image burning to demonstrate the path of a laser.

The same idea works for documenting chemiluminescence. Instead of tracing the laser trajectory,

the chemiluminescence is burned first in a dark room. The lens is then covered with a cardboard

again and the flash is used to paint in the background. This technique allows both the reaction and

the room to be clearly seen in the photograph.

OVERVIEW OF DIGITAL PHOTOGRAPHY

Up to this point, this guide has focused on traditional photography. Traditional

photography is an analogue process where light is focused on a photo-sensitive emulsion which

results in a recorded image. It is considered analogue because the image is recorded with a

continuous scale of tones from light to dark. In digital photography our eyes perceive a continuous

scale of tones, but the actual image is recorded as a number of discrete picture elements known as

pixels. Together these pixels form the image that we see. If you continue to enlarge a digital image,

the pixels eventually become visible.

Figure 41: While the digital image on the right seems to be a continuous gradation of tones, enlargement will reveal pixilation.


29

In film, the size of the silver halide crystals that record the image corresponds to the resolution.

Similarly in digital imaging, the number of pixels used to record the image corresponds with

resolution. Since the number of pixels is determined by the camera, it becomes an essential way to

describe a digital camera. Any time reference is made to a digital camera, one of the first things

established is how many millions of pixels or megapixels it is capable of recording. The first digital

camera developed as a prototype in the early 1970s was capable of achieving 0.01 megapixels.

Current digital SLRs are capable of over 12 megapixels.

While the number of pixels correlates to resolution, it clearly is not the only factor that

affects it. The quality of the lens is a significant factor in evaluating resolution. Just like traditional

film, digital cameras can range from “point-and-shoots” to sophisticated SLRs and even digital

backs for medium and large format cameras. As a result of lens quality, a 3.1 megapixel “point-and-

shoot digital camera might have lower resolution than a 2.4 megapixel interchangeable lens camera.

Figure 42: These two photographs illustrate how the camera lens can affect resolution. The photograph on the left was taken with a 3.1 megapixel built-in lens digital camera. The one on the right was taken with a 2.4 megapixel interchangeable lens digital camera. (Courtesy SWIGIT and the FBI) Computer software also affects the resolution. To better understand this, it is necessary to

have a rudimentary understanding of how a digital camera works. The framework is essentially the

same as a film camera. A lens will focus the image on a photosensitive material, in this case an

image sensor. There are several different types of image sensors available, but the most common is

known as a charge coupled device (CCD). The sensitivity of the image sensor is adjustable and is

described as the ISO or film speed (even though it is not film anymore). The amount of light that is

allowed to strike the image sensor is controlled by the physical size of the lens opening (f #) and by

how long the shutter stays open (shutter speed). Exposure is calculated in the same way. The

difference comes in how the image is recorded. Unlike film where the exposure is complete after it


30

30

strikes the photosensitive material, in digital this is the beginning of a process. The image sensor

records the image that strikes its surface and then takes it apart, moves it to a storage device and

reconstructs it all via a computer algorithm.

Figure 43: Cross-section of a modern digital SLR

More sophisticated software does a better job of maintaining the resolution and integrity of the

image as it is taken apart and reassembled. Most digital cameras will allow the photographer to

adjust the resolution for each shot. Higher resolution images have more information and tend to

move through the virtual information pipeline a little slower than smaller files. Frustrated

photographers that cannot wait for the image to be recorded might decide to turn the camera off

and on during this process to try and “reset it.” When they do, they risk significant damage to the

file, the media storage device, and even the camera. The software feature that controls how much

information is stored while it is being written to the media storage device is known as the buffer.

Another feature of the software is something known as the burst rate. The burst rate relates to how

many images you can take in rapid succession without the camera having to pause and catch up

while the camera finishes writing the files to the storage device. Larger files have a smaller burst

rates than smaller files so one way for a photographer to overcome the burst rate limitation is to

reduce the resolution.

There are many different options recording digital media. The media storage device also has

software associated with it that can affect the images recorded onto them. Sometimes this can cause

compatibility issues, and the cameras manual should always be consulted when purchasing media

storage devices.


31

Figure 44: Three examples of different electronic media storage devices currently being used.

Another difference between film and digital has to do with the difference in detection

methods. Because of the instrumental nature of the light detection mechanism, in addition to

traditional reciprocity failure issues for short exposure, digital is also subject and signal to noise

issues for long exposures. Extremely long exposures may be obscured by noise before they will be

affected by reciprocity.

The image sensor introduces another variable. For the most part, the size of the image

sensor is not the same as the size of a 35 mm film plane. This difference means that we have to

reevaluate normal perspective based upon the size of the detector. Usually the normal perspective

for a digital camera is about 35 mm.

Digital cameras also allow you to choose how you want to store the pictures files. There are

three options: they can be stored as uncompressed files which take up the most space, they can be

saved as lossless compression files which take up slightly less space, or they can be saved as lossy

compression files which can dramatically reduce the size of the file. Uncompressed files are

recorded as RAW files in proprietary software provided by the manufacturer. A lossless

compression is a computer algorithm that reduced the size of a file by reducing redundant

information in the file, but doing so in such a way that it can be reproduced and no information is

lost. If a picture is taken of a large grassy field in the spring, the image sensor might see 50 pixels

worth of green information: gggggggggggggggggggggggggggggggggggggggggggggggggg. Another

lossless way to represent this information is to say 50g. Examples of lossless compression files are

Tagged Image File Format (TIFF), Graphic Interchange Format (GIF) and Bitmaps (BMP). The

classic example of a lossy compression is a Joint Photographic Experts Group (JPEG) file. JPEGs

can have varying degrees of compression, but all of them lose information that was originally in the

file. Repeatedly saving a file as a JPEG will result in the continued loss of data.


32

32

CHOOSING A PHOTOGRAPHIC SYSTEM

In the same way that there is no one good condition to take a photograph, there is no one

good photo system to take it with. The determination has to be made based upon the intended

application. It is essential that an agency or individual fully define what they intend on doing and

what level of training the individual or individuals using the photo system possess.

Some general guidelines can be followed regardless of the application. One lens never fits all

regardless of platform. The photographer should be able to change the lenses. The camera should

be able to support a flash off of the camera. This means that a wire from the camera to the flash

should be able to transmit metering and flash synchronization information to the flash yet still allow

the photographer to move the flash to alter the angle of illumination or prevent reflection. The

camera should be able to accommodate a shutter release cable. Certain scene and laboratory

techniques require the shutter to be operated in the bulb mode using a remote cable.

Resolution is often a concern and guidelines are application specific. The Scientific Working

Group on Image Technology (SWGIT) was formed to make recommendations regarding a host of

issues in forensic photography and does so regarding resolution. The SWGIT guidelines can be

found on the International Association for Identification’s website:

www.theiai.org/guidelines/swgit/index.php. For the most part, the recommendations are common

sense. If photographs are going to be used to examine fine detail, the bigger the negative the better.

While evidence such as footwear impressions and tire marks can be photographed with 35 mm, they

do better in medium format and would be even better in large format. Even the best digital SLRs

are not recommended for this type of evidence. The National Institute for Standards and

Technology (NIST) recommends a minimum resolution of 1,000 pixels per inch for latent print

evidence. This standard also seems reasonable for footwear and tire marks. Modern digital cameras

are capable of achieving this resolution, but only for a relatively small field of view.

Figure 45: Examples of digital camera field of views. (Courtesy SWIGIT and the FBI)


33

If a digital camera was all that was available, an examiner could determine the distance from the

surface to the camera lens that is needed to achieve this resolution. Then the examiner can use a

tripod to maintain this distance with overlapping photographs all taken at the field of view needed to

get 1,000 pixels per inch.

While there are many scientific methods of evaluating if a digital camera is up for a certain

task, a more pragmatic approach is called for. The most effective way to address this is to determine

the smallest feature that you are trying to discern in the photograph. Set up a test with the smallest

item in the test. If you can discern it, then the technology is acceptable. If not, either 35 mm or

medium format is called for.

If you are choosing a digital camera, you also need to be concerned with the camera’s

software and media storage device. For example if a 6.1 megapixel camera is only capable of

recording images in a JPEG format, it will not actually deliver 6.1 megapixels of resolution. It will

record 6.1 million pixels worth of information, but some of that information will be lost due to lossy

compression. The camera will make recommendations on which storage devices are compatible.

These recommendations should be followed to prevent loss of data.

AUTHORS NOTE

Using a camera today is a little like using a computer, everyone has and uses one, but few of

us really know or understand how to use more than a few limited applications. This guide is simply

designed to expand a forensic scientist’s use of photography to document physical evidence. Even

with all of the information presented in this guide, no mention has been made of developing or

printing film, processing digital images or archival concerns. Taking the pictures is the first critical

step in the process. Even if nothing else is gained through reading this guide, increasing the quality

of this critical step would be a worthy accomplishment.

ACKNOWLEDGMENTS

I would like to thank Keith Freitag of the Newark, New Jersey FBI Field office and Michael

Brooks of the FBI Photo-training unit in Quantico, Virginia for their technical review of this

manuscript. I would also like to thank Stacy Ann Collins and Jacqui Hudson of Pace University for

their assistance. Most of all I would like to thank my wife, Carol, as well as my two sons, Zachary

and Nicholas, for their enthusiastic support and love.


34

34

BIBLIOGRAPHY Bodziak, W. Footwear Impression Evidence – Detection, Recovery, and Examination, 2nd ed., CRC Press, New York. Busselle, M., The Art of Photographic Lighting, David & Charles, Devon, UK. 1996 Davis, P., Photography, 7th ed., McGraw-Hill, New York. 1995. De Forest, P, Gaensslen, R, & Lee, H., Forensic Science – An Introduction to Criminalistics – Appendix 3, Fundamentals of Photography, McGraw-Hill, New York, 1983. Grimm, T, & Grimm, M., The Basic Book of Photography, Plume, New York. 2003 Kodak Publications: AC-95 Guide to 35 mm Photography B-3 Photographic Filters Handbook M-2 Using Photography to Preserve Evidence M-27 Ultraviolet and Fluorescence Photography M-28 Applied Infrared Photography R-27 Gray Cards London, B., Upton, J., Kobre, K, & Brill, B. Photography, 7th ed., Prentice Hall, New Jersey, 2002. SWGIT Guidelines: www.theiai.org/guidelines/swgit/index.php Voder Bruegge, R., Imaging in Forensic and Criminology, from the Encyclopedia of Imaging Science and Technology, John Wiley, New Jersey, 2002

a forensic scientist's guide to photography brian j

Documents