Digital Low-Light Photography
Written by Sanford Weiss   

Digital Low-Light Photography

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.

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 many forensic evidence documentation situations, the level of existing light (ambient illumination) is sufficiently high to enable the capture of properly exposed, high-resolution, sharply detailed, handheld images. Bright lighting allows the use of reasonably fast shutter speeds, proportionally small lens apertures, and enables the proper operation of the camera’s auto-focus capabilities.

To be appropriate for hand-held exposures, the level of ambient illumination must be high enough to set a fast shutter speed that will minimize the movement of both the camera and the subject to a negligible level during the exposure. On the other hand, three things determine the slow exposure-speed threshold for creating sharp 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 length or telephoto lenses will amplify camera movement during exposure and may require faster shutter speeds or the use of a tripod.

If you want to produce images with negligible blur from camera movement, first determine the slowest shutter speed recommended in that situation for handheld exposures. This can be estimated by looking at the actual focal length of the lens. The actual focal length of a lens on a DSLR may be calculated by multiplying the real focal length by the crop factor of the sensor. The focal length of lenses from 35mm cameras may be effectively longer when used on digital cameras due to the smaller size of the image sensor. The crop factor of a Nikon D80, for example, is 1.5. A 105mm Nikkor lens on a film camera is essentially a 157mm lens on the D80. Therefore, the slowest recommended shutter speed for a non-VR (vibration reduction) 105mm Nikkor on the D80 is 1/157th of a second. (To ensure sharp images, it may be better to err on the side of safety and use 1/250th rather than 1/125th for the base shutter speed.) Image stabilization technology provided in some lenses may allow the use of shutter speeds three to four times (f-stops) slower—or exposure times eight to 16 times longer—than with non-stabilized lenses under the same conditions.

Exposure

The factors that affect the sensitivity of digital sensors are efficiency and capacity. Sensors with greater efficiency and higher capacity

HERE’S A HANDY TIP

When 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 overexpose the images to compensate.

for light capture are rated with higher ISOs. If the ISO setting of a sensor is too low for the use of shutter speeds sufficiently fast for the scene’s ambient illumination, then the photographer must compensate by exposing images at a slower shutter speed in order to create images with sufficient contrast for full documentation of the subject.

A numeric exposure value (EV) represents a combination of shutter speeds, apertures, and ISO sensitivities that yield equivalent exposures. For example, an EV of -1 at ISO 100 corresponds to a shutter speed and aperture combination of 15 seconds at f/2.8, or 4 minutes at f/11, etc. An EV of 19 at ISO 100 corresponds to 1/1000th of a second at f/22, or 1/250th of a second at f/45, etc.

Each unit increase in EV corresponds to a change of one f-stop in exposure. When the EV goes up 1, half the exposure is required, and when the EV goes down 1, twice the exposure is required. Exposure compensation is achieved by changing the shutter speed, lens aperture, ISO sensitivity, or a combination of any of these three factors.

Bright lighting is the most appropriate illumination for the capture of most images. However, the high contrast and dark shadows inherent in brightly lit scenes may require the use of an electronic flash, reflector, or other light sources to lighten the shadows and reduce the subject contrast range. This will depend on the dynamic range capture capabilities of the image sensor in the camera.

Low levels of ambient illumination may interfere with the camera’s ability to capture images with accurate color, show objects and scenes with a recognizable level of inherent dimensionality, or with reasonable DoF.

The picture elements (pixels) that make up a digital-camera sensor contain light-sensitive photodiodes that convert the amount of incoming light at each pixel location into a proportional electrical signal. During image processing, the electrical signal is converted to a value corresponding to the color of that particular pixel in the final image. Conversion errors, for a number of reasons, may cause a lack of color uniformity in the final image called noise. Visual noise is most noticeable in monochromatic image areas. The noise level varies per pixel, depending on exposure time, and it increases proportionally to the elevation of the sensor temperature and ISO. The temperature of the image sensor goes up in proportion to the amount of work performed by the camera to capture a given image. When ambient illumination is low, the time required for the capture of sufficient and proper exposure increases, raising the sensor temperature 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 the ISO sensitivity of the media must increase. As ISO increases, relative resolution of the image decreases due to rising levels of image contrast and visible noise. When the ISO level is too high, the lens aperture is too large, or the shutter speed is too slow, the resulting images may have a number of problems: insufficient resolution, depth of field, or sharpness. In order to properly document the evidence in these situations, low-light imaging practices must be employed.

Electronic Flash

As was true with film photography, digital cameras will capture images in most any range of ambient illumination. In some low-light situations, a photographer may be forced to use supplementary lighting—most often some form of electronic flash unit (EFU). The addition of electronic flash illumination to a scene may achieve sufficient exposure, enabling the documentation of details over a wide range of colors, contrasts, and distances. In many situations, the use of an EFU is both accepted and expected for the creation of forensically acceptable images. EFUs are limited in effective range proportionally to their size and output capacities. Most small or built-in EFUs will illuminate areas up to 15 to 25 feet from the camera, depending on the situation and environment.

The guide number (GN) for an electronic flash defines its output strength. The GN may be calculated for either feet or meters. The guide number allows the photographer to calculate the maximum distance at which a flash may yield proper exposure for a given ISO sensitivity and subject. The GN is divided by the lens aperture to derive the distance. For example, if the GN of a particular EFU is 80 (in feet) at ISO 100 and at f/4, the maximum effective distance from the flash to the subject will be 20 feet.

The photographer must be aware that the use of an EFU in low light may create detail inaccuracies in the resulting images. To capture images that most accurately resemble the content and contrast of a low-light scene, it may be necessary to utilize only the ambient illumination for exposure. However, it is not unusual to be required to document large indoor or outdoor scenes at night, when supplementary lighting may be insufficient or inappropriate.

Ambient Illumination

The contrast between objects allows the human brain to differentiate between the objects it perceives. If an object is conspicuous

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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.

against its background, it will be easily noticed. Any perceived contrast includes differences in tone or color, depending on the level of ambient illumination. To photographically produce images that a viewer will perceive as accurate representations of the subjects in low light, the subject’s brightness range, contrast, and conspicuity must often be documented through use of only the ambient illumination.

The contrast of a scene is determined by the light level and angle of illumination. As the light level decreases, the perception of colors, shapes, and forms decreases. If the angle of the illumination source approaches zero degrees (in other words, directly above the subject), shadows cease to exist and subjects become less conspicuous. Color vision does not exist when light levels are sufficiently low, although black and white vision is still possible. The limitation of color vision in very low levels of illumination may cause a person with normal color vision to perceive a red object identically to a gray object.

Remember, a white object may appear yellow, orange, or green when viewed under mercury vapor or carbon arc illumination.

After sufficient exposure in low light, the image created may or may not be a reasonably accurate representation of what the viewer perceived with his or her eyes, depending upon how the image was created. Most outdoor night scenes are predominantly dark with small areas of illumination from windows, signs, headlights, reflections, and streetlights. Aesthetically pleasing photographs may be exposed using a variety of techniques over a wide range of exposures. Short exposures will preserve highlight detail in well-lit areas. The shadows will appear dark due to underexposure. Longer exposures will enhance detail in the shadows, while highlight detail may be lost due to overexposure.

Scenes that are predominantly dark or backlit are difficult to meter accurately with a camera’s built-in exposure meter even when it is set to register only illumination from a small spot. It may be necessary to perform light-meter readings from the position of the camera with a hand-held or built-in spot meter or from the position of the subject using a hand-held meter with incident capabilities.

 

Handling backlit subjects

All ambient light has an inherent color tonality and is categorized using a Kelvin (K) temperature scale. Sunlight is a different color than florescent or incandescent (Tungsten) light. Unless a light-source is filtered to be strictly red, green, blue, or yellow, the human eye will perceive the light combination only as brightness, and may not be able to differentiate its color balance and components. Digital cameras will produce images with a color cast that matches the color balance of the ambient illumination unless the camera is set to compensate its white balance (WB). Most digital cameras include defaults for Tungsten, florescent, and daylight, as well as automatic. Some cameras will set a WB preset using a white subject or reference under a specific light source. Some cameras will set the K to a specific temperature value chosen by the photographer. Any of these options allows additional control of the WB and aids in the creation of accurate color images under most lighting conditions. In mixed lighting, set the camera to approximate the predominant light-source temperature.

The metering range of the Nikon D80 is EV 0 to 20 in 3D color matrix and EV 2 to 20 in spot mode at normal operating temperature, ISO 100 and f/1.4 lens. The normal operating temperature for most digital cameras is between 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 photographer saw at the time of the exposure. If it is necessary for the photographer to perform a broad range of post-processing procedures to the images, then capture in RAW mode is usually recommended.

Use of a Tripod

When the exposure time is longer than normal, the photographer must put the camera on a stable platform during the exposure. Always try to use a tripod that is heavy enough to keep the camera steady at its set height. A tiny tripod will work if the camera is set on the roof of a car, with the engine turned off. A heavy camera, a windy day, or very long exposures will call for heavy tripods.

Expectations

Evidence photographers always strive to create low-light photographs that accurately document scenes as they would have been seen by the victim(s) of an occurrence. Many techniques have been presented and arguments made to prove the accuracy of photo-graphs. Here is the bottom line: It is truly impossible to create images of scenes as they would have looked to another person at another time. A short list of factors prohibiting the capture of images to represent reality includes the following:

  • The limitations of photographic media including lens focal length, and field-of-view;
  • Environmental conditions including weather, vegetation, pavement reflectance, and so on;
  • Recreation of exact lighting conditions from street lamps, businesses, passing traffic, and so on;
  • Cleanliness of headlights and windshields;;
  • Physical condition of victim;
  • Degree of adaptation of the vision of the victim to the ambient illumination;
  • The difference between instantaneous and extended vision

Conclusion

Capture images of low-light scenes using whatever technique provides images that are easily explainable to the intended audience. It is acceptable to state that the images created are reasonably accurate representations of how the scene looked to you, the photographer, at the time of the photography. Include a photograph of a test target in one image of the project. Record the section of the chart with the lowest contrast that is visible to you in the subject lighting. When viewing the final image in low lighting—similar in value to the subject scene—the same section of the target should be differentiable.

To say that the final images represent an accurate representation of what another person may have seen is a very dangerous practice for a forensic photographer. An opposition expert must only prove one element of difference between the images and reality to discredit your work.

About the Author

Sanford Weiss is the author of a book published in 2009 by Pearson, Prentice Hall. The title of the book is Forensic Photography: The Importance of Accuracy. Weiss can be reached by e-mail: This e-mail address is being protected from spam bots, you need JavaScript enabled to view it


ORIGINALLY PUBLISHED:
"Digital Low-Light Photography," written by Sanford Weiss
September-October 2009 (Volume 7, Number 5)
Evidence Technology Magazine
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