terminology - What is a normal lens?

I've heard people call 50mm lenses "normal lenses".

For a lens to be considered normal, does it have to be exactly 50mm or is there some leeway? Does the normal focal length depend on crop factor of body used with lens? Are there any important relationships with other lens categories, such as wide, telephoto, prime, zoom, standard lenses?

Answer

First, there is nothing magical about the 50mm focal length. A 50mm is a "normal" lens only on the 135 format ("FX"), 24x36mm full frame. On a slightly smaller sensor like the APS-C format it is a short portrait "tele" (as used colloquially meaning a "long" lens, not in the exact technical sense of the word where the focal length is longer than the physical length), on a larger sensor like Hasselblad-style medium format film it will be a wide-angle. And on a tiny mobile phone sensor a 50mm lens would be a preposterously long ultratele that you wouldn't want to use without a heavy-duty tripod to keep it steady as a rock. Focal length must be viewed relative to sensor size.

The strict definition I've seen of a normal lens is that its focal length should be equal to the diagonal of the sensor... which obviously isn't quite true for a 50mm lens on 135-sized film. But never mind that, there is obviously some slop in the definition, anything from 45mm-ish to 60mm-ish has been marketed as the "normal" lens for various 135 film camera systems and there is nothing wrong with that I suppose. For that matter, a "50mm" lens isn't usually exactly exactly 50mm in the first place, it can just as well be 48.5 or 51.3mm if you look hard at it. The main point of a "normal" lens as I understand it is that it is a lens that is neither noticeably wide-angle nor noticeably tele; in other words a lens that sees the world pretty much as the eye sees it, so that a photo taken with it looks... normal. I used a Soviet SLR camera with a 58mm lens on it once, it gave a wonderful 1:1 magnification in the viewfinder so I could shoot with both eyes open without the least mismatch between what my left and right eye could see. Can it get more "normal" than that? (Of course, this depends on the exact magnification in the viewfinder as well as the lens!)

A normal lens is a prime lens, by definition. It can be made as a telephoto lens (in the technical sense) if the lens designer absolutely wants to, but there is usually no particularly good reason to make it one - unless one wants to make a pancake lens out of it I suppose. It is neither wide nor tele (in the non-technical sense), by definition.

Back in the day, before zooms became commonplace, most 135 SLR cameras were sold with a 50mm prime as the el cheapo kit lens, which made the 50mm the lens that everybody and their grandmother had. Many of the very popular fixed-lens rangefinders from the fifties through the seventies had 50mm lenses, although models with a slightly wide 40mm lens were also very common. Contax and Leica interchangeable lens rangefinder cameras, which go way back to the late twenties and early thirties, also came with 50mm lenses as the default choice and actually needed extra bolt-on viewfinders to be used with any other focal length. So it would not be very wrong to call the 50mm the "standard lens" throughout much of photographic history, at least for 135-format consumer cameras.

A cousin of the normal (prime) lens is the "normal zoom", which is the moderate-wide-angle to moderate-tele kind of zoom you usually get as a kit zoom. Typically 28-70mm-ish on full frame, 18-55ish on APS-C. The normal zoom covers the focal length that would be used for a normal (prime) lens and can zoom a bit wider and a bit longer than that.