Yep, another technical post! We’ve looked at aspect ratios, now we’ll look at ideal focal lengths.
As you may know, 50mm is intended to be the ideal focal length for 35mm photography. It provides roughly the same angle of view as the human eye. Why is this so? Well to work out the ideal focal length of the format, you just work out the diagonal length of the format. Why is this so? I’m not too sure, honestly, but it’s what I’ve read.
So 35mm is 24×36.
Let’s apply some good ol’ Pythagorean theorem:
The length of the diagonal side of a right angle triangle is equal to the square root of the sum of the square of the remaining two sides.
sqrt((24^2)+(36^2))
sqrt((576)+(1296))
sqrt(1872)
=43.26
The diagonal of 35mm is a 43.26, which would be closest to a 42mm lens. So now you see why 50mm is seen as semi-telephoto by some people, and why anything from 35mm, 40mm or 50mm is considered the ideal focal length. It’s personal taste. 35mm is just as close as 50mm, but wider.
But what about some other formats?
6×6/56×56: 79.2/80mm
6×7/56×70: 89.64/90mm
6×8/56×80: 97.65/100mm
6×9/56×90: 106/105mm
4×5/101.6×127.0: 162.64/150mm
So in medium format, the diagonal seems to be pretty close to the generally agreed upon focal length. In 4×5, 150mm is considered about standard, but now it all gets open to interpretation again. Keeping in mind that these measurements aren’t exact and can/do differ from camera to camera, and through my mistakes.
There you go, some more stuff you didn’t need to know but now do.