Originally posted by Class A There is a relationship in that there is an assumption that one will look at an image from a distance that corresponds to the image diagonal.

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Thanks for the pointer. I haven't read the whole article yet but it starts off in an odd way.

I am sure we're having a hundred year old discussion but it's fun

First, let me repeat that the "normal" focal length is not purely about a concept. The normal focal length is one where, if you look with one eye through the VF and with another one directly, both images would overlap. Some (few) photographers really do this to frame their images...

E.g., a Pentax MX (0.97x magnification with 50mm) and Pentax ME and LX/FB-1/FD-1 (0.95x magnification) use 52mm as this normal. With the K-7, it's 54mm (**).

Do we really look at an image at ~diagonal distance? I'll have to observe this

At least as for watching TV, it seems to be wrong. E.g., a 50" screen is typically watched from 2.5m to 4m viewing distance which is exactly 2-3x the diagonal. Official recommendations vary between 3-5x. I think 3x is the smallest where no head turns are required to capture all parts of an image with equal attention.

2,3,4x the diagonal is 28°,19°,14°.

**3x** with

**16:9** is 16.5° x 9.3° (

**16°x9°**) which is really really easy to remember

According to the nice link to the Leica article, the human's field of view spans 200° x 140° (240° diagonal if the field of view were rectangular).

So, the human vision is better characterized by a 200° (8mm) FishEye combined with a 14° (180mm) Tele.

I find it challenging to define the "normal" for a 8-180mm

*double-focal* lens. This is why I was asking.

At least, the geometric mean between 8mm and 180mm is 38mm which is close enough to the image diagonal rule. Still, looks rather arbitrary to me. Which is why I said I would appreciate any "official" source for the "50mm is normal" tradition.

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(**) If I try it with K-7+60mm (easiest in portrait orientation) I find that the K-7 image is indeed almost exactly as large as the naked eye vision. The published 0.92x magnification is meant without any crop factor correction (54mm for overlapping images).

The double-eyed composition is really cool. I see black frame borders in the middle of the entire scene which I am still continuing to see. Unfortunately, the large camera body obstructs one half of it. An electronic viewfinder clipped onto one eye (monocular) could be a cool gadget