Visual Expert Human Factors: Photographs vs. Reality

Marc Green

"All photographs of any kind are always distorted relative to reality."
- R. M. Evans. Using photography to preserve evidence. Kodak Publication M-2, 1976.

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:

The amount of light in the court room will alter apparent brightness;
The level of light to which the viewer has just been exposed (adaptation state) will affect brightness and contrast;
Any differences in eyesight between the viewer and the person involved in the accident can change perception;
The visual size (i. e., visual angle) of objects in the film will likely differ from those at the scene. Size affects visibility greatly;
The court will look straight at the photo (using the highest acuity part of the eye), while accident the participant may have been looking with the peripheral visual field (where acuity is lower);
Still images miss many of the important visual cues that people use, such as motion parallax (motion of one object relative to another); and
Photos also lose the visual cues, i .e., binocular disparity and texture.

Cognitive Reasons

I have discussed these in the Computer Animation Has Perceptual Limitations page, so I'll just briefly list the most important here:

There are many psychological biases in interpreting images. Blurred and low contrast objects appear farther away, objects on the left of an image often appear closer than objects on the right, etc.
Perception involves knowledge and expectation. The court cannot put itself in the mind of the accident participant
Perception is a function of time. The court can casually study the photos. In an accident, the participant is usually caught off-guard and must respond in a split-second glance.

Summary

Photographic images should not be used as graphic representations of reality for many reasons. There is, unfortunately, no real substitute for going to the accident scene, measuring luminances of object and background, comparing these values to visual contrast threshold norms and then taking into account visual and cognitive factors. This procedure is partially described in Determining Visibility.

Lastly, these comments apply only to the use of photographs as representations of what a person would have seen. Photographs may serve other valid purposes to aid the court in finding of fact. However, any visual image can create subtle cognitive effects that create bias.

Other Topics

Personal Injury: Road Accidents Is The Moth-Effect Real? Human Error in Road Accidents Reaction Time Let's Get Real About Perception-Reaction Time Vision in Older Drivers Weather and Accidents: Rain & Fog Accidents At Rail-Highway Crossings Seeing Pedestrians At Night Underride Accidents Rear End Collision: Looming Night Vision Distracted Pedestrians Failure To See	Personal Injury: Warnings & Product Defects Warnings and Warning Labels Warning Effectiveness Checklist The Psychology of Warnings Drugs, Adverse Effects & Warnings Are Warnings Effective? Human Error Vs. Design Error Product Misuse And "Affordances" Safety Hierarchy: Design Vs. Warning Thinking Like A Human Factors Expert
Personal Injury: Other Diving Accidents in Pools Falls Down Steps Medical Error Computer & Medical Error Nursing Error	Criminal & Police Errors in Eyewitness Identifications Perceptual Error in Police Shootings Eyewitness Memory Is Unreliable Human Factors In Forensic Evidence
Intellectual Property "Any Fool Can See The Trademarks Are Different" Measuring Confusion For Intellectual Property Color in Trademark and Tradedress Disputes Color Functionality: A Case Example	Forensic Human Factors Seeing Color Determining Visibility "Inattentional Blindness" & Conspicuity Computer animation has perceptual limitations Photographs vs. Reality The Six Laws Of Attention What is "inattention?"