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Clearing DNA Backlogs

The magazine dedicated exclusively to the technology of evidence collection, processing, and preservation

Volume 7, Number 5 • September-October 2009

TOPICS IN THIS ISSUEn Automating latent-print examination

n Securing the P&E room

n Screening tools for suspicious powders

n Digital low-light photography

G O T O P A G E 1 F O R D E T A

Clearing DNA Backlogs



Evidence Technology Magazine • September-October 2009www.EvidenceMagazine.com

24

THE PRESENCE OF LIGHT—or another alternative form of energy—is required by any film

or digital camera in order to capture

images on sensitive media. In manyforensic evidence documentation situ-ations, the level of existing light(ambient illumination) is sufficiently

high to enable the capture of properlyexposed, high-resolution, sharplydetailed, handheld images. Bright

lighting allows the use of reasonablyfast shutter speeds, proportionallysmall lens apertures, and enables theproper operation of the camera’s

auto-focus capabilities.To be appropriate for hand-held

exposures, the level of ambient illumi-nation must be high enough to set afast shutter speed that will minimize

the movement of both the camera andthe subject to a negligible level dur-ing the exposure. On the other hand,three things determine the slow expo-sure-speed threshold for creatingsharp images: 1) the capabilities of the photographer, 2) the movement of the subject, and 3) the focal length of the lens. The use of long focal lengthor telephoto lenses will amplify cam-

era movement during exposure andmay require faster shutter speeds orthe use of a tripod.

If you want to produce imageswith negligible blur from cameramovement, first determine the slow-est shutter speed recommended in

that situation for handheld exposures.This can be estimated by looking atthe actual focal length of the lens.The actual focal length of a lens on aDSLR may be calculated by multi-plying the real focal length by thecrop factor of the sensor. The focallength of lenses from 35mm camerasmay be effectively longer when usedon digital cameras due to the smallersize of the image sensor. The cropfactor of a Nikon D80, for example,is 1.5. A 105mm Nikkor lens on a

film camera is essentially a 157mmlens on the D80. Therefore, the slow-est recommended shutter speed for anon-VR (vibration reduction) 105mmNikkor on the D80 is 1/157th of asecond. (To ensure sharp images, itmay be better to err on the side of safety and use 1/250th rather than1/125th for the base shutter speed.)Image stabilization technology pro-

vided in some lenses may allow theuse of shutter speeds three to fourtimes (f-stops) slower—or exposuretimes eight to 16 times longer—thanwith non-stabilized lenses under thesame conditions.

ExposureThe factors that affect the sensitivityof digital sensors are efficiency andcapacity. Sensors with greater effi-ciency and higher capacity for lightcapture are rated with higher ISOs. If the ISO setting of a sensor is too lowfor the use of shutter speeds sufficientlyfast for the scene’s ambient illumina-tion, then the photographer mustcompensate by exposing images at aslower shutter speed in order to cre-ate images with sufficient contrast forfull documentation of the subject.

A numeric exposure value (EV)represents a combination of shutterspeeds, apertures, and ISO sensitivitiesthat yield equivalent exposures. Forexample, an EV of -1 at ISO 100 cor-responds to a shutter speed and aper-ture combination of 15 seconds at f/2.8,or 4 minutes at f/11, etc. An EV of 19at ISO 100 corresponds to 1/1000thof a second at f/22, or 1/250th of a

Digital Low-Light PhotographyWritten by Sanford Weiss

This street scene was exposed wih a NIKON D70S, built-in EFU, ISO 400, WB auto, 28MM lens, autofocus at 1/60 second, F5.6. The image is a combination of flash and ambient illumination. It was exposed by Sanford Weiss, San Francisco, California.



P H O T O G R A P H Y

Evidence Technology Magazine • September-October 2009 25www.EvidenceMagazine.com

second at f/45, etc.Each unit increase in EV corre-

sponds to a change of one f-stop inexposure. When the EV goes up 1,half the exposure is required, andwhen the EV goes down 1, twice theexposure is required. Exposure com-

pensation is achieved by changing theshutter speed, lens aperture, ISO sen-sitivity, or a combination of any of these three factors.

Bright lighting is the most appro-priate illumination for the capture of most images. However, the high con-trast and dark shadows inherent inbrightly lit scenes may require theuse of an electronic flash, reflector,or other light sources to lighten theshadows and reduce the subject con-trast range. This will depend on thedynamic range capture capabilities of

the image sensor in the camera.Low levels of ambient illumina-

tion may interfere with the camera’sability to capture images with accu-rate color, show objects and sceneswith a recognizable level of inherentdimensionality, or with reasonableDoF.

The picture elements (pixels) thatmake up a digital-camera sensor con-tain light-sensitive photodiodes thatconvert the amount of incoming lightat each pixel location into a propor-tional electrical signal. During image

processing, the electrical signal isconverted to a value corresponding tothe color of that particular pixel inthe final image. Conversion errors,for a number of reasons, may cause alack of color uniformity in the finalimage called noise. Visual noise is

most noticeable in monochromaticimage areas. The noise level variesper pixel, depending on exposuretime, and it increases proportionallyto the elevation of the sensor temper-ature and ISO. The temperature of the image sensor goes up in propor-tion to the amount of work performedby the camera to capture a givenimage. When ambient illumination islow, the time required for the captureof sufficient and proper exposureincreases, raising the sensor tempera-ture and increasing the possible level

of noise.To create reasonably accurate

images of a subject in low levels of ambient illumination, the length of the shutter speed and the level of theISO sensitivity of the media mustincrease. As ISO increases, relativeresolution of the image decreases dueto rising levels of image contrast andvisible noise. When the ISO level istoo high, the lens aperture is toolarge, or the shutter speed is too slow,the resulting images may have anumber of problems: insufficient res-

olution, depth of field, or sharpness.In order to properly document theevidence in these situations, low-lightimaging practices must be employed.

Electronic FlashAs was true with film photography,

digital cameras will capture imagesin most any range of ambient illumi-nation. In some low-light situations, aphotographer may be forced to usesupplementary lighting—most oftensome form of electronic flash unit(EFU). The addition of electronicflash illumination to a scene mayachieve sufficient exposure, enablingthe documentation of details over awide range of colors, contrasts, anddistances. In many situations, the useof an EFU is both accepted andexpected for the creation of forensi-

cally acceptable images. EFUs arelimited in effective range proportion-ally to their size and output capaci-ties. Most small or built-in EFUs willilluminate areas up to 15 to 25 feetfrom the camera, depending on thesituation and environment.

The guide number (GN) for anelectronic flash defines its outputstrength. The GN may be calculatedfor either feet or meters. The guidenumber allows the photographer tocalculate the maximum distance atwhich a flash may yield proper expo-

HERE’S AHANDY

TIPWhen using an electronic flash in low light to capture images of objects with reflective markings (including law-enforcement and fire-suppression vehicles), the high reflectivity of the safety markings on the subjects will cause more than the normal amount of light to bounce back to the automation sensor of the flash.

This may cause the EFU to turn off too quickly and create under- exposed images.

It may be necessary for the photographer to intentionally overex- pose the images to compensate.



register only illumination from asmall spot. It may be necessary toperform light-meter readings from theposition of the camera with a hand-held or built-in spot meter or fromthe position of the subject using a hand-held meter with incident capabilities.

All ambient light has an inherentcolor tonality and is categorized usinga Kelvin (K) temperature scale.Sunlight is a different color than flo-rescent or incandescent (Tungsten)light. Unless a light-source is filteredto be strictly red, green, blue, or yel-low, the human eye will perceive thelight combination only as brightness,and may not be able to differentiateits color balance and components.Digital cameras will produce imageswith a color cast that matches thecolor balance of the ambient illumi-

nation unless the camera is set tocompensate its white balance (WB).Most digital cameras include defaultsfor Tungsten, florescent, and daylight,as well as automatic. Some cameraswill set a WB preset using a whitesubject or reference under a specificlight source. Some cameras will setthe K to a specific temperature valuechosen by the photographer. Any of these options allows additional controlof the WB and aids in the creation of accurate color images under mostlighting conditions. In mixed light-ing, set the camera to approximatethe predominant light-source temper-ature.

The metering range of the NikonD80 is EV 0 to 20 in 3D color matrixand EV 2 to 20 in spot mode at normaloperating temperature, ISO 100 andf/1.4 lens. The normal operating tem-perature for most digital cameras isbetween 32° and 95°F.

Image Modes

The two most common image modes

available for digital exposures are

JPEG and RAW. Either will produce

images that are reasonably accurate

representations of what the photogra-

pher saw at the time of the exposure.

If it is necessary for the photographer

to perform a broad range of post-pro-

cessing procedures to the images, then

capture in RAW mode is usually rec-

ommended.

Evidence Technology Magazine • September-October 2009www.EvidenceMagazine.com

26

sure for a given ISO sensitivity andsubject. The GN is divided by thelens aperture to derive the distance.For example, if the GN of a particu-lar EFU is 80 (in feet) at ISO 100and at f/4, the maximum effectivedistance from the flash to the subject

will be 20 feet.The photographer must be aware

that the use of an EFU in low lightmay create detail inaccuracies in theresulting images. To capture imagesthat most accurately resemble thecontent and contrast of a low-lightscene, it may be necessary to utilizeonly the ambient illumination forexposure. However, it is not unusualto be required to document largeindoor or outdoor scenes at night,when supplementary lighting may beinsufficient or inappropriate.

Ambient IlluminationThe contrast between objects allowsthe human brain to differentiatebetween the objects it perceives. If anobject is conspicuous against itsbackground, it will be easily noticed.Any perceived contrast includes dif-ferences in tone or color, dependingon the level of ambient illumination.To photographically produce imagesthat a viewer will perceive as accuraterepresentations of the subjects in lowlight, the subject’s brightness range,contrast, and conspicuity must often

be documented through use of onlythe ambient illumination.

The contrast of a scene is deter-mined by the light level and angle of illumination. As the light leveldecreases, the perception of colors,shapes, and forms decreases. If theangle of the illumination sourceapproaches zero degrees (in otherwords, directly above the subject),shadows cease to exist and subjectsbecome less conspicuous. Colorvision does not exist when light levelsare sufficiently low, although black

and white vision is still possible. Thelimitation of color vision in very lowlevels of illumination may cause aperson with normal color vision toperceive a red object identically to agray object.

Remember, a white object mayappear yellow, orange, or green whenviewed under mercury vapor or car-bon arc illumination.

After sufficient exposure in lowlight, the image created may or may

not be a reasonably accurate represen-tation of what the viewer perceivedwith his or her eyes, depending uponhow the image was created. Mostoutdoor night scenes are predominantlydark with small areas of illumina-tion from windows, signs, headlights,reflections, and streetlights.Aesthetically pleasing photographsmay be exposed using a variety of techniques over a wide range of exposures. Short exposures will pre-serve highlight detail in well-lit areas.The shadows will appear dark due to

underexposure. Longer exposureswill enhance detail in the shadows,while highlight detail may be lost dueto overexposure.

Scenes that are predominantlydark or backlit are difficult to meteraccurately with a camera’s built-inexposure meter even when it is set to

P H O T O G R A P H Y

When visible light is totally absent, a sufficient level of exposure to form an image on sensor or film cannot be achieved unless the camera sensor or film is sensitive to other wavelengths. Monochrome images may be exposed using infrared light under appropriate conditions.

HERE’S ANOTHER

HANDYTIP

Handling backlit subjects

ETM V7N5 Photography

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