Archived posting to the Leica Users Group, 1997/08/11
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Mon, 11 Aug 1997 15:30:17 -0400 (EDT) Alf Breul wrote: >In einer eMail vom 11.08.1997 18:54:55, schreiben Sie: > >>I always thought that the "normal" lens was one where the field of view >>most closely matched that of the human eye. For the 35mm format I >>understood this to be a 35mm lens, for 4x5 a 6" lens. The 50mm focal >>length for the 35mm format is a slight telephoto. My "normal / standard" >>lens is a 35mm f/2 Summicron. >> >> > >If you take those standards, then you are almost true: the 50 mm covers the >angle of the human eye, while the 90 mm covers the perspective of the human >eye. So, which is your standard? > >The optical standard for a "normal" lens is given by the equation I sent >recently (in short: the standard normal lens agrees to the length of the >diagonal of the negative format). - ---------and Mon, 11 Aug 1997 02:43:49 -0400 (EDT) Alf Breul wrote: >There is a usual computation for the standard focus: It's the root out of >2*(square of larger side of negative format), e.g. 36 mm is the larger side >of the 35 mm format, then > >standard = root of (2 * 36**2) = root of (2*1296) = 50.9117 (mm) > >or for MF: > >MF = root of (2 * 60**2) = root of (2*3600) = 84.8528 (mm) > >which shows also the known fact, that the usual MF standard focus tends to be >a (small) wide angle. 'Standard' focal length has, I know, been stated to be the length of the diagonal of the film format. The formula given above is of course something completely different, and additionally for most 6x6 cameras the actual format is 56mm x 56mm (being _really_ picky), giving about 79.2mm as a diagonal. So 80mm is about as close as anybody is likely to get. On the other hand, 'standard' focal length, if calculated in this fashion, should really be left in the textbook. If you stand facing grand mountain (or other) panorama for the first time, you tend to use as your 'normal' vision part of your peripheral vision, as well as the almost automatic scanning function of your eyes. 'Wideangle' is natural for this type of situation. After you have looked at the scene for a while, you focus on details, and eventually try to make out things at the limit of your visual ability. 'Long Focus' is 'normal' now. I don't have any data in front of me on this topic, but the human eye can highly resolve only a very tiny angle, certainly a lot less than a 90mm or even 135mm lens on 35mm takes in. Anything beyond that has strong qualitative falloff. On the other hand, real life does not have a mat around it, except in special circumstances. Usually, we can scan 360 degrees in all directions if we please, letting our brain do the 'post-processing' to take care of distortion issues and take in as much detail as we want (up to the limits of our visual acuity). We always have wideangle and telephoto built in, so what is standard? I think in the end a 'normal' or standard focal length is one that shows that angle of view of a subject that looks natural to us; for scenics and architecture that often means that the picture was taken with a wideangle lens, and for portraits and other details that means that the picture was taken with a long focus lens. If we see a picture which was taken with a focal length that shows an image perspective that is definitely counter to our usual interaction with that type of subject, then the picture was taken with a 'non-standard' focal length. My $0.02Cdn :-). * Henning J. Wulff /|\ Wulff Photography & Design /###\ henningw@archiphoto.com |[ ]| http://www.archiphoto.com