Common photography myths (4)

Myth #3: A 50 mm lens on 35 mm systems is called "normal" because it delivers about the same view as the human eye

September 5th, 2009 - 08:32:23 PM:

You have already guessed it, this isn't really the case. Human vision is much different from that of a camera. The horizontal angle of view is about 120 to 140 degrees, which translates to 13 mm to 9 mm focal length with a 35 mm camera. However, we don't see very sharp over this entire field of view but only at the center. This part of our retina has an angle of view of about 60 degrees, which translates to 35 mm focal length. The area with the sharpest vision, the one that you use right now to read these lines, is even smaller. Its angle of view is only 2 degrees, equivalent to 1100 mm focal length.

The lateral field of vision is mainly used to detect motion while the central part is used for precise visual inspection. So you see, human vision is quite different from the view through a 50 mm lens.

Some other theory says it's the standard focal length because it's the diagonal size of the 35 mm frame [and even that isn't precisely true; the diagonal size is about 43.27 mm]. Well, that's how we calculate what the standard focal length for a given format is, but it's not the reason why it was actually selected as the standard.

The real reason is much more mundane. When SLRs (or rather: system cameras) became widely availably to a large number of amateur photographers, the makers of these systems had to select one lens that they could sell as a standard set together with the camera. Following the logic of economics, it had to be one that could be made well for little money. 50 mm lenses fit these criteria perfectly. They typically are focused by extension (simple mechanism), don't require aspheric elements (simple lens element shapes), they don't require elements made of glass with anomalous dispersion (simple materials), and they're not zooms (fewer elements and simpler mechanism). It's easy to make a really good and fast 50 mm lens for little money. Also for this reason, the 50 mm standard lenses are often among the best lenses of one maker's lens lineup. That's why most 35 mm cameras came with a 50 mm lens until recently. The makers could have also selected 40 mm or 55 mm as the standard, but 50 mm probably looked more “even”.