Why did a 68mm lens on full frame look most like my eye, not a 'normal' 50mm?
Asked 3/24/2020
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I understand focal length, angle of view, and sensor size, but I’m confused about what “normal focal length” really means. The common rule says a focal length close to the frame diagonal gives a natural view, so on full frame that suggests about 43–50mm. But when I looked through my Sony full-frame camera and zoomed my 28–70mm lens until the viewfinder matched what I saw with my other eye, it lined up closer to 68mm. Does that mean my perception is unusual, or am I mixing up different ideas about what “normal” means?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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The original phrase was "normal angle". The earliest reference related to photography that I could find dates to 1909. Earlier references are mostly related to medicine. One photographic reference to a "normal lens" in 1893 was referring to an unmodified lens.
When the focal length is equal to the diagonal of the frame, the angle of view (of the diagonal) is ~53.4 degrees. The angle of view of a 50mm lens on 135 film is ~46.8 degrees.
Whether the image seen in the viewfinder matches that of the photographer's eye depends on the magnification of the viewfinder.
Points in Focus: Depth of Field (DoF), Angle of View, and Equivalent Lens Calculator
Complete Self-instructing Library of Practical Photography: Negative retouching, etching and modeling
James Boniface Schriever
American School of Art and Photography, 1909
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
6y ago
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You’re mostly mixing up two different things: a “normal” lens and viewfinder magnification.
A normal focal length is a historical/photographic concept: roughly a focal length near the format diagonal, which gives a perspective and angle of view that generally looks natural when the final image is viewed at a typical size and distance. On full frame, that’s around 43mm, and 50mm is commonly treated as normal.
What you tested was different: you adjusted the lens until the apparent size in the viewfinder matched your unaided eye. That result depends heavily on the camera’s viewfinder magnification, not just focal length. So it’s completely possible for a full-frame camera to “match” your eye at something like 58–68mm in the finder.
In other words, your eyes are probably fine. The mismatch comes from comparing finder magnification to normal angle/perspective. “Normal” does not mean “the exact same apparent size as my eye when looking through this particular viewfinder.”
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