How to View a Photograph, 101


How to View a Photograph, 101
Photographs of accident scenes, especially those made specifically to depict the view of a motorist, pedestrian or eyewitness, are sometimes very helpful to the trier of fact in understanding how and why an accident has occurred. An important consideration when viewing a photograph for such purposes is the correct distance between the observer and the photograph, or the correct viewing distance. At the correct viewing distance, objects in the photograph will be the same optical size as observed from the vantage point of the photographer.

This is to say, the photograph viewed at the correct viewing distance will be essentially the same scene as if the viewer were standing at the location the photograph was made and looking in the direction the camera was pointed. (An exception to this rule would be the use of wide-angle lenses that will be discussed in a subsequent article.)

The correct viewing distance for any photograph is simple to calculate, but requires specific knowledge of the focal length of the camera lens and the extent to which the original negative was enlarged. The formula to determine correct viewing distance (cvd) is: cvd = photo enlargement ratio x lens focal length.

Example: A 35 millimeter camera with a 50 millimeter (normal range) lens was used to make an image that was printed at a full frame enlargement size of 24 inches x 36 inches. The enlargement ratio is simply a ratio of the size of the photo enlargement, 24 inches x 36 inches, to the size of the negative image, 24 x 36 millimeters. Using the width of the image, and converting from metric to English units, the enlargement ratio is:

Next, convert the focal length of the lens to inches: 50 mm x 0.0394 inches/mm = 1.97 inches. Finally: cvd = 25.4 x 1.97 = 50 inches

In comparison, the correct viewing distance for 4 x 6 inch and 8 x 12 inch prints of the same image would be 8.3 inches and 16.7 inches respectively. Please note that from a human factors perspective, many people cannot resolve the details of a 4 x 6 inch print at 8.3 inches, since they can't focus this closely. Therefore, 8 x 12 inch prints are recommended as a minimum size.

-Gary Hesler

Reprinted from The Reconstructor, Newsletter of Boster, Kobayashi & Associates.
Volume 1, Issue 2, Spring 1999.

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