Why is a 50mm FF lens 'normal'?

Started Feb 24, 2013 | Discussions thread
jrtrent
Veteran MemberPosts: 3,417
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Re: Perspective effects.
In reply to Barrie Davis, Feb 27, 2013

What did you think of the article Tom Axford linked to above (I think I referenced an abstract of that paper in a thread about 3 months ago)?

http://forums.dpreview.com/forums/post/50922975

They said that the viewing distance needed for perspective to appear natural (as opposed to compressed or exaggerated) was equal to the focal length of the lens times the magnification needed for the print size (resulting in what they called the center of projection).  They also noted that people tend to choose a viewing distance based on the size of the print, not the CoP, and that it happens that what most people find a comfortable viewing distance for various size prints works out to be fairly close to the CoP for a normal focal length lens (e.g., 50mm on a full frame camera).  Hence their conclusion, included in the abstract:

"We found that people's preferred viewing distance when looking at pictures leads them to view long-focal-length pictures from too near and short-focal-length pictures from too far. Perceptual distortions occur because people do not take their incorrect viewing distances into account. By following the rule of thumb of using a 50-mm lens, photographers greatly increase the odds of a viewer looking at a photograph from the correct distance, where the percept will be undistorted."

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