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?
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.
A normal lens is one who's focal-length is equal to the diagonal of the sensor or film. This is said to give a natural perspective similar to that of a single human eye.
On a full-frame DSLR, it is usually a 50mm lens. On a cropped-sensor (APS-C) DSLR, a normal lens falls around 35mm but from 30 to 55mm, it would still be considered normal. For Four-Thirds and Micro Four-Thirds, you would use a 25mm. Usually most manufacturers make sure to have one bright prime that corresponds to the normal focal-length for the sensor-size.
50mm was called a normal lens for 35mm films because the field of view was approximately that of the human eye.
Small digital sensors usually have a magnification factor applied so that their normal lenses would be somewhat smaller (some examples). I'm sure more experienced commenters can fill in more details but I think it's the field of view that would be important.
I have a couple of ancient 35mm. cameras which have what I believe to be "normal lenses". One is a Voiglander CLR rangefinder with a fixed 50 mm 2.8 and the other is a Nikkormat FTN SLR with a 50mm 1.4. With those old cameras, when I brought the veiwfinder up to my right eye, I could keep my left eye open and everything would be brought into focus without any double vision etc. Back in the day the viewfinders were 100% with a 1.0x magnification. What you saw is what you get. It is a pretty neat way to view and shoot things this way.
I cannot find any modern camera which allow me to do this (at least in the consumer range that I can afford). These days with the crop sensors and <1.0x viewfinder magnification and the need to fit additional digital information into the view finder, good luck trying to keep your left eye open while shooting. I get double vision and a headache.
In the context of the modern camera. for me a "normal" lens is more of a subjective personal choice based upon my perspective. When I see a scene with my naked eye, I want to take a picture from my perspective and I choose a focal length which produces a photo which bests matches up with what I saw with the naked eye. In other words, I want to use a focal length which does not require me to zoom in or out with either the lens or my feet. For me, on my crop sensor Canon Rebel, that focal length is usually within the approximate range of 24mm to 28mm, and sometimes 35mm. For a full frame 35mm, that would translate to (1.6 x) approximately 38mm to 45mm, up to 56 mm. This seems to be consistent with some articles which state that 43mm to 50mm is the normal perspective for a full frame. I personally prefer the wider end. It is always better to include more in the photo and crop later than vice versa.
I have been wanting to buy a 24 or 28 mm prime for my Rebel for the longest time, but could not justify spending the money. I am pretty excited about the upcoming Canon EF-S 24mm 2.8 which will retail for only about $150. If the IQ is good, I will finally pick up what for me would be the ideal lens.
No, it doesn't have to be exactly 50mm. I use a Voigt 40mm/F2 pancake lens instead of the 50mm. The 40mm is just perfect. Shot vertically it looks like a very slight wide angle shot with no field relevant distortion and critically sharp...
My understanding of a "normal" lens as a casual student dipping in and out of books and online courses is that 50mm lenses (as found on 35mm film cameras) replicates the same focused field of view as the human eye. Which also just happens to be approximately 50°. Though it is true that we see a much wider angle than that in our peripheral vision, how much of it is actually in focus? The thing about the 50mm "normal" lens theory, is that generally speaking, if you was to put a rectangular frame in front of you, showing only a 50° field of view, what you see is what you will capture on a camera with that lens focal length. No "compression", no "widening" and no "distortion". When it comes to different camera types, crop factor must be taken into account to achieve this "normal" effect. I have a Nikon D3300 with an APS-C sensor which has a crop factor of 1.5x meaning to get the equivalent 50mm effect, I must divide 50 by 1.5, giving me 35mm. Which just so happens to be the focal length of their prime (fixed length-non zoom) lens for "normal" photography. Digital street photographers will often use a 35mm (or equivalent conversion from 50mm depending on the crop factor of their sensor), so that no matter what distance the are from a subject, the picture will render true to life with no curving due to optical distortion of a wide angle lens etc. And a little leeway is fine :-) photography isn't a black and white science ;-)
It is a common mistake to describe "normal" lenses by comparing focal distance to sensor or film diagonal - a normal lens shows the proportions like the human eye. Tele lenses compress them, and wide angle lenses expand them, as you take the proportions of far and near objects. 50mm is about the average for the human eye and it changes nothing if your camera crops or not. Don't mix up "normal" and "prime". It is the "prime" lens which changes as you change the image diagonal size. For a 6x6 reformat camera a 85mm is a prime, while for an APS-C it is about 35 mm and a phone it is about 1.5 cm or so. But "normal" is 50 mm where it draws distance proportions as a human eye.
