Photographs vs. Reality
"All photographs of any kind are always distorted relative to reality."I'm writing this section because of several calls that I've recently received from attorneys. In each case, the attorney (or expert) had taken photographs at an accident scene and wanted me, as an expert, to confirm that the photographs could be presented to the court as a valid representation of what someone would have seen at the time of the accident. I've had to decline because they cannot. Several reasons are discussed in "Computer Animation Has Perceptual Limitations." Here, I elaborate on a few more points and explain further why photographs and animations cannot be used for this purpose. The reasons fall into three categories, physical, sensory and cognitive. Physical Reasons Photographic film simply does not record light "veridically," i. e., truthfully. Most notably, it distorts light intensity (brightness) and wavelength (color). Film is non-linear, meaning that the response does not increase proportionally with light intensity. For example, if the scene doubles in light intensity, the photograph will not record double the light. The same is true for color - it does not record wavelength faithfully. The amount and type of distortion depends on the exact film and the way that it is developed. Moreover, film has a small "dynamic range" relative to the eye. This means that it compresses the range of darkest dark to the brightest bright, so that contrast is generally lower than reality and objects are less visible. All of the problems are inherent in digital cameras, videotape, and video displays, too. Lastly, cameras also distort brightness because of the way they determine exposure, as I explain elsewhere. The photo appearance will also be greatly affected by choice of film, aperture size, development and especially exposure duration. With color film, longer durations make scenes look brighter and short duration make them darker. Film can integrate light over an extended period while the eye integrates light for about a tenth of a second (depending somewhat on adaptation level.) Camera optics produce other distortions. Lens choice can have a large effect on spatial relations. Short focal length (wide angle) lenses tend to cause 3D spatial relations to distort because the lens is typically closer to the subject. Conversely, long lenses (telephoto) tend to compress objects together. The angle of the photograph also affects relative size of objects at different distances. Lastly, there are common perspective errors. When the image of the 3D world falls on a 2D plane, such as the eye or a camera film, it creates perspective relative to a particular viewpoint called the "center of projection." For an accurate and undistorted perception, the viewer's eye must be properly located at this center of projection point in space while inspecting the image. To be viewed accurately, jurors would have to be placed at the correct eye position to view the images. Sensory Reasons In theory, some film nonlinearities, and possibly range compression, could be corrected to accurately show the scene brightnesses and color accurately. This is an involved process and, in any event, could not compensate for several sensory factors (reduced dynamic range, for example), as well as the cognitive problems discussed below. Sensory problems occur because photograph are easily distorted vs. reality by visual factors at the time they are viewed:
- R. M. Evans. Using photography to preserve evidence. Kodak Publication M-2, 1976.
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