High Dynamic Range in Crime Scene Photography
Written by Sandy Weiss & Arthur Borchers   

PHOTOGRAPHY CANNOT BE ACCOMPLISHED without illumination. Thus, the key variable in any photograph, even more than composition, is the type and amount of lighting used to create the image. A subject in total darkness cannot be photographed by visible light no matter how hard the photographer tries. If the image is being exposed by just the ambient visible light, there must be some illumination, and some contrast, to capture the image. If there is even the slightest illumination and contrast, a shutter speed of sufficient length, an aperture of sufficient circumference, and an ISO speed set to sufficient sensitivity on the camera, then an image of the subject in question may be captured in sufficient detail to discern most or all of the subject elements.

Dynamic range is the difference between the brightest and darkest areas in a photograph. Most DSLR cameras have a dynamic range capability of eleven to thirteen stops. A stop, from F-stop, is also called an exposure value (EV) and has a precise relationship. Each full value increment is exactly double or half of the original (i.e., ISO 400 is twice as sensitive as ISO 200; 1/500 is half the shutter speed of 1/250; and f/8 allows half the amount of light as f/5.6).

The contrast of the subject may be compressed if the illumination level is low. In a situation like this, most any camera with manual settings may be used to capture the image with no problem or worry on the part of the photographer that the dynamic range of the subject image is beyond the capabilities of the camera. There is no black, there is no white, and most probably no colors—only relative shades of gray. Once successfully captured, the image made by the camera should be easily printed by a printer with a normal dynamic-range output capability, or the image may be seen on a monitor or projected by a normal projector with no issues of image detail loss due to being too dark or too bright.

Now, consider a subject in the other end of the illumination scale, when the given subject has almost too much illumination. The lighting of a mid-day sun can be so bright, the dynamic range of the subject so wide, that it may be difficult to capture an image of the subject and retain full detail in both its brightest highlights and darkest shadows. Situations like this may also arise when using artificial illumination like electronic flash at a fire scene or on a roadway at night. It is quite possible for parts of the scene to be black and others to be white. The problems arising in this case in capturing an image with full detail at both ends of the illumination scale may be because the scene is beyond the capabilities of the equipment, or the scene is beyond the capabilities of the photographer.

Every part of the imaging process has its own limitations and capabilities. The camera may certainly be out of its depth when capturing images at a fire scene in bright sunshine. If the digital images do capture the whole dynamic range of the subject, the printer used to make reflective images of the subject may not have the dynamic range capabilities to output images without constricting the true dynamic range of the subject. There is no possibility for the printer to print a white brighter than the color of the paper being used to make the prints. There is no possibility for the printer to print a black blacker than the maximum blackness designed into its capabilities. And even if everything is working properly up to this point, every time an image is shared, there is the possibility that a link in the imaging process of someone else’s lab may not have sufficient capability to output an accurate representation of the subject in question.

So, this is the important question: Will the final image be accurate? Is the dynamic range of the image being submitted an accurate representation of the original subject? It is up to the photographer, as the first link in the imaging chain, to do everything possible to capture images of the subject to be taken into court and be testified to as the best and most accurate representations of the original subjects possible to be captured.

Histogram

In what ways may the photographer ensure success in capturing accurate images? The first and simplest aid to accuracy is the use of the image histogram. This graph of the dynamic range of the photograph is easily accessible with any camera having manual settings. Consider the histogram to be a graphic representation in Cartesian coordinates of the captured image. Every time an image is made, the accompanying histogram is available to be viewed as a menu option. The bright pixels of the image are graphed on the right side of the X-axis, and the dark on the left. Everything in between is the representation of the mid-tones. Each peak in the histogram on the Y-axis is a graphic representation of the number of pixels of that particular brightness in the captured image. If an image of a subject is captured in bright sunshine, or with highly reflective white subject elements, and the graph of its pixels on the right side does not rest at zero on the Y-axis, it means the highlight detail of the subject is incomplete, or clipped, in the photographic representation. Conversely, if the graph of the dark pixels on the left side is not at zero on the Y-axis, it means the shadow detail of the image is clipped. Either way, the image is not complete and therefore not totally accurate.

One way to attempt greater accuracy is to take a light-meter reading in the subject illumination off of an 18% gray card and use that reading as the mid-point of a manual exposure. But even after this routine, it is still possible to capture images with clipped details in the shadows, the highlights, or both.

Is there a way to ensure the optimum capture of dynamic range in any photograph under any lighting situation? This is where HDR imaging takes over from straight photography.

Admissibility

When images are brought to court—civil or criminal—the party entering them into evidence must show they are relevant to the issue at hand and tend to prove or disprove a material fact in the case, e.g., the existence of skid marks or lack thereof. Someone who was present at the scene (not necessarily the photographer) must authenticate that the photograph fairly and accurately represents the conditions, person, or scene it depicts. Each photograph introduced must be authenticated, sometimes ad nauseum. Questions about who took the photo, how it was taken, and other matters generally do not go towards admissibility questions unless some special technique was used, such as HDR, focus stacking, or the use of infrared or ultraviolet lighting. In those situations, the person applying the specialized technique must testify to the basis of the technique to show it is reliable.

A photographer must be cognizant that a digital photograph does not fit the traditional definition of an original photograph. In the film days, an “original” image was the result of light being allowed to shine on a layer or layers of sensitive silver crystals bonded to a media like acetate or glass. Then the image was developed by chemical processes that revealed a positive or negative duplicate of the focused light image allowed through the lens. If reflective prints were necessary, copies could be made by photographing the originals and chemically processing the duplicates the same as the first.

A digital photograph starts in the same way, where a focused light image strikes a digital sensor that replaces the film. The sensor measures the color value and intensity of the light that falls on a grid of tiny picture elements (pixels) that number in the millions (megapixels) and are considered a measure of camera quality as the number of pixels increases. The sensor then sends the information to the camera’s microprocessor which then writes all the image’s information to a file on a memory device, such as a secure digital (SD) or compact flash (CF) card. The file name relates to the sequential order in which the image was taken and is written following exchangeable image file format (EXIF) rules agreed upon by camera manufacturers. The EXIF format includes metadata about each photograph, including camera information, exposure data, and even GPS coordinates that can be examined by looking at the file properties. The file extension, such as JPG, will designate the rules followed in creating the image and how the data file was decoded and viewed as an image on a computer screen, or printed on paper. The traditional piece of film that could be held in a hand and viewed by holding it up to a light source no longer exists. The data file should be transferred to secure storage media as soon as possible. The major difference between digital- and silver-based photography is that the original data file, once moved from the memory card to a hard drive, is still considered an original. Pains must be taken not to modify the original image file, so generally any post processing work is done on a “working copy.” A copy of a digital photo does not require a new photo to be taken, just a copy and paste command on a computer.

RAW v. JPEG

What’s the difference? All digital cameras capture RAW images and then internally process the images according to the settings on the camera. These settings include White Balance, Saturation, Contrast, Sharpening, and Color. The final setting applied is Quality which compresses the color values from 12-bit (4096) increments per color channel to 8-bit (256) increments per color channel and squeezes the double-digit megapixel resolution of the camera’s sensor down to an image grid of 4352 pixels by 3268 pixels or smaller. This reduction in resolution may be perfectly adequate for day-to-day photographs but, when faced with challenging lighting or situations where detail is critical, keeping the RAW image becomes necessary.

Be warned that a RAW image is significantly larger in size than even a very high-quality JPEG. Agency SOP should address both the use of RAW images and storage. For example, JPEGs may be stored in a secure photo drive but RAW images may be saved to CD-R or DVD and inventoried separately. RAW images are generally proprietary to the camera maker (Nikon’s “NEF” or Canon’s “CR2” formats). Adobe Systems has proposed a digital negative (DNG) standard which has not been as well received as their portable document format (PDF).

HDR Imaging

HDR photography has its origination in artistic photography but has value for evidentiary purposes. In the early days of film photography, the tonal range of the printed images produced was normally extremely limited. In the 1850s, Gustave LeGray used two negatives to capture a seascape—one for the sky and a second longer exposure for the water—to present an image to mimic the luminance levels seen by the eye. Later, Ansel Adams used special film exposure, developing techniques and selective print exposure methods called dodging and burning to expand the tonal range of prints produced.

However, color photography eliminated most of the creative darkroom techniques used with black-and-white film and prints. Color-film manufacturers designed new films with increased range and sensitivities. When digital imaging started, Adobe Systems created Photoshop and, later, Lightroom software packages used to manipulate digital images. Those who may be hesitant to use or accept any form of photo editing should be aware that the International Association of Identification approves of the practice with the caveat that any manipulation be done on a working copy of the original image and that any process be documented so it is repeatable and the results consistent if someone else tries to replicate the modifications.

In HDR photography, only the shutter speed is adjusted between exposures in order to maintain a constant depth of field across the captured images. HDR photos are usually captured with the camera mounted on a tripod to minimize movement between exposures. Visible movement within the images is called “ghosting.” If the image contains plants or water some movement is inevitable. The available software packages typically compensate for ghosting by taking object outline information from the image with the fastest shutter speed to freeze movement and filling in tonal information taken from the other exposures.

The major software tools for HDR imaging are Photoshop (Adobe Systems) Photomatix (HDRsoft), and Aurora (MacPhun). Note: Aurora has previously been a Macintosh-only program but the Aurora HDR 2018 iteration is also being released for Windows OS.

All three systems are capable of producing court-ready images. Some HDR photo modifications are so extreme as to be almost unrecognizable when compared to the original. But any forensic image needs to be faithful to the original scene. The technician needs to be ethical and knowledgeable about any manipulations and modifications. A good example of lack of ethics is that it is possible to enhance specific colors like reds in a bloody photo to increase the shock effect.

The important part of the process is to know when the image to be captured will not work as desired. This knowledge is only obtainable through study and practice. If the image highlights are too blown out or the shadows too flat, that is a good opportunity for HDR to assist. Modern DSLR cameras typically have an Auto-Exposure Bracket (AEB) function that will automatically take three, five, or even seven bracketed photographs in sequence with an exposure difference the photographer defines. For three exposures, ±2-stops is usually sufficient. If the camera does not have AEB, then it is necessary to switch to manual mode, determine the correct average exposure for the scene, and take the first photo. Then input a 2-stop faster shutter speed for the second exposure and a 2-stop slower shutter speed for the third. The manual method will certainly take more time and likely have camera movement, so a tripod will be a necessity. Use of a single photo to produce an HDR image from a normal image is possible but not necessarily prudent. After all, image details which do not exist in the original are normally not possible to display with enough difference to the original image just by selective manipulation of the same image.

The image samples for this article were taken in the Chicago Transit Authority’s LaSalle Street Blue Line station and show part of the track crossover. The exposures were shot at ISO 3200 with a Canon G1X camera. The images were shot with a ±2 stop AEB at f/8 with shutter speeds of 1/5, 1/20, and 8/10 seconds with both RAW and JPEG files created simultaneously.

This is the standard, normal exposure:

This is the -2-stop exposure:

This is the +2-stop exposure clearly showing camera shake:

In order to convert the images in Photoshop, the three images were selected together in Adobe Bridge and the Tools > Photoshop > Merge to HDR Pro command selected. The images were then sent to Photoshop where they were converted to individual layers and the Remove Ghosts checkbox was clicked. None of the options to modify the image tone were adjusted and the OK button pressed. The resulting HDR image was saved and is shown below:

Some ghosting is still visible in the signs at the right, but overall there is more detail visible in the shadow areas.

In Photomatix, the same three files were selected in the workflow window. Then the HDR Merge options were selected: alignment and cropping with allowance for hand-held, perspective correction, noise and chromatic aberration reduction. Below are two rendered samples demonstrating basic presets. The first with “Enhanced Detail” and the second with “Contrast Optimized.”

Next, another image was created using the newly released Aurora HDR 2018 from the same three RAW files, with minor tweaking of color and sharpness.

The final image was created using the Canon RAW images. You will note the inclusion of the signal lights at the right and the full second alcove at the left. These were cropped out by the camera’s 13:9 image setting for the JPEG, but the entire 4:3 image was still recorded in the RAW version. No color correction was made.

The time involved in creating each image is minimal but the benefits of HDR imaging can be substantial.


About the Authors

Sandy (Sanford) Weiss is the author of Forensic Photography; the Importance of Accuracy, Published by Pearson, Prentice - Hall. He retired from active field-work in 2012 but continues to keep up with developments. He published several articles in Evidence Technology Magazine over the years.

Arthur Borchers is currently an adjunct instructor for the Suburban Law Enforcement Academy at the College of DuPage and a Forensic Consultant with Larsen Forensics & Associates, both in Glen Ellyn, Illinois, having advanced training and experience in photography, photogrammetry, firearms, shooting incident, crime scene and traffic crash reconstruction after having retired from the Oak Park Police Department in 2013.


 

This article appeared in the Winter 2017 issue of Evidence Technology Magazine.


 

 

 
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