Originally posted by rparmar My definition is that it is a lens with the same focal length as the diagonal of the film/sensor. Thus 43mm is normal for full-frame and 28mm on APS-C.
This definition has several advantages.
1. It has nothing to do with perspective, since focal length doesn't.
2. It has nothing to do with ideas of how the human eye sees, since a camera works nothing like an eye, human or otherwise.
3. It corresponds to historical reality, eg. what camera manufacturers in all formats have always told us was normal. Yes, 43mm is off from 50mm, but the normal lens in different formats has always scaled with the film size in this way.
4. It corresponds with equivalent field of view, which is what most people really care about when they choose a focal length.
Right on! And why is everyone so hung-up on normality? Or normalcy, as one authority called it. Be abnormal. Be as abnormal as possible, as abnormal as you can get away with, without being detained involuntarily for long periods. CHERISH your abnormalcy. WORK WITH your abnormalcy. And watch YOUNG FRANKENSTEIN.
But I digress. Problem is, as with so many other words, 'normal' has different meanings in different contexts. It's a right-angle, 90 degrees (from the Latin
norma, a carpenter's square). IIRC, it's within one standard deviation of a mean/average (statistics). In photo marketing, it's whatever lens is supplied with the camera (unless it's marketed as a wide or tele cam). It's mainstream, not-too-weird (in uptight societies). It's being free from disease (medicine, biology). I'm sure other definitions can be found.
And in photographic optics, it's the diagonal of a frame -- but frames aren't all the same aspect, so the AOV provided by that diagonal varies. APS-C and 35/HF and 35/FF and 645cm and 6x9cm all have a 2:3 aspect ratio; their normals have the same AOV. A 4x4cm or 6x6cm or any other square frame is 1:1, with a smaller AOV. The HDTV 16:9 aspect has a greater AOV. Anything beyond that is definitely wide; but we could legitimately refer to the diagonals of aspects between 1:1 and 16:9 as 'normal'.
rparmar is absolutely correct. Can we go on to other subjects now?
My suit is pale yellow, my nationality is French,
and my normality has often been subject to question.
--T.Williams