Logo Computer scientist,
engineer, and educator
• Articles • Articles about graphics and illustration


The hills in the distance look close together, but this is an optical illusion caused by the relative closeness of the foreground objects
 
 

The myth of perspective compression

Photographers — or more often photography publications — frequently make the claim that the use of telephoto lenses will 'compress perspective'. I've never been entirely clear what is meant by that; I presume that the desired effect is to make objects that are distant from each other appear closer together. In fact, there is no such thing as perspective compression. The significance of the telephoto lens is that it allows the photographer to stand further from the subject Hardly magic, but visually relevant all the same.

This article was prompted by something I read on another Web site, in which a photographer tried to disprove the myth of perspective compression by taking a series of pictures at different zoom settings while standing in the same place. He then edited the photos so that each showed the central subject at the same size and — wonder of wonders — they all looked the same. There is, the author claimed, no such thing as perspective compression.

That's true, of course, so long as he was standing in one spot. All he was doing in a test like this was to use the computer to compensate for the variation in field of view of the zoom lens. There might be some small variation in depth of field across the images but, otherwise, we'd expect them to look much the same.

The two diagrams below are intended to show the relationship between lens focal length (or, rather, angle of view), impression of perspective, and photographer position. In figure 1, the photographer is using a lens with a 30-degree field of view. If the viewpoint is such that the trees fill the frame, then the trees are relatively separated, compared with their distance from the viewpoint. Features of the front tree that are the same size as those of the back tree would appear roughly twice as large in the photographer, simply by virtue of the photographer's close viewpoint.


Figure 1: with a wide-angle lens the photographer must stand close to the trees (the subjects, in this case ) for them to fill the frame. Compared to the distance from the photographer to the subjects, the subjects are far apart

Figure 2: with a telephoto lens the photographer can stand some way from the subjects and still have them fill the frame. The subjects are close, relatively speaking, compared with their distance from the photographer's viewpoint.
In figure 2, however, the field of view is only 15 degrees. The trees now fill the frame with the photographer's viewpoint much further away than in figure 1. Unless the trees were actually directly in line (so that the viewer could see that one was in front of the other), they would appear to be about the same distance away in the photograph. That's because features on the two trees would now appear to be about the same size in the photograph.

We might talk of the second photograph as showing 'compressed perspective', but all that's really happened is the the camera has been placed in such a way as to make features of the subjects appear similar in size. Perspective compression is an illusion, caused by the way our brains process size and distance cues.

And that's all there is to it. You can't adjust the impression of perspective simply by fitting a lens with a longer focal length. The effect on perspective comes from changing your viewpoint.

Comments welcome, as always.

Copyright © 1994-2013 Kevin Boone. Updated May 15 2012