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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?

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Also relevant: Why is the 50mm prime lens the most standard? and Why do people recommend 50mm...? ... lots of answers but IMO none very convincing of what normal/50mm really means. –  DHall Jan 18 '12 at 20:16

4 Answers 4

up vote 8 down vote accepted

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.

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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.

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Why 35mm for APS-C? Isn't 28mm the size of the diagonal? (Or more like 27mm for Canon?) –  mattdm Jan 18 '12 at 15:14
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Measurements seem very loose when it comes to these things :) It is normally quoted as 35mm because 50mm / 1.5 = 33mm and 35mm is the closest common focal-length. If your calculation is correct, 50mm wont be the diagonal of a full-frame sensor either! –  Itai Jan 18 '12 at 15:37
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It isn't; it's closer to 43mm. Clearly there's some free range allowed. I've heard 40-55mm as the "normal range" on 35mm, which would correspond to about 26mm to 35mm on APS-C. I wonder what the standards are for older and larger formats. –  mattdm Jan 18 '12 at 16:05
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645 is actually 56×41.5, its diagonal is 69.7 mm and 75 mm normal lenses for this format are available. The Pentax 645D is 44×33, diagonal is 55 mm and its normal lens is actually a 55 mm. 6×6 is really 56×56, its diagonal is 79.2 mm and it commonly uses 80 mm as a normal focal length. –  Edgar Bonet Jan 19 '12 at 9:24
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50mm was always long for a "normal" lens. On a 6x6 (56mm square), the normal was 80mm, 90mm was the "normal" on a Mamiya 6x7 (56mm x 70mm), on a 4x5 it was 150mm or 135mm (depending on the expected extension), and 210-300mm on 8x10. The common (and cheap) 2-inch cine lens was appropriated for use on the early 35s (like the Ur-Leica), and it just sort of stuck. Considering that the standard print was usually a 4x5 or an 8x10, a 38mm lens would have been more "normal" (and less available in the early days). –  user2719 Jan 20 '12 at 8:00

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.

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Really? The field of view of my vision seems much, much wider than that. –  mattdm Jan 18 '12 at 15:19
    
Your total field of view is about 180 degrees, obtainable only with a fisheye lens on a camera! Most of it is peripheral vision though, which the brain doesn't "see" in any great detail. –  Staale S Jan 18 '12 at 15:57
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Let me quote Wikipedia on normal lens: “A lens with a focal length about equal to the diagonal size of the film or sensor format is known as a normal lens; its angle of view is similar to the angle subtended by a large-enough print viewed at a typical viewing distance equal to the print diagonal”. There is an interesting nuance here: it's not really the field of view of the human eye, but rather the field of view that feels comfortable for seeing a photographic print. –  Edgar Bonet Jan 19 '12 at 9:32

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.

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Really? So if I take an Olympus Stylus 9010 and zoom it to its maximum focal length of 50mm, the proportions and tele-compression will be the same as with a 50mm lens on my film camera, even though a 9mm focal length on the small-sensor compact digicam gives the same framing without moving my feet? –  mattdm Jan 18 '12 at 19:50
    
And are you saying that this isn't a prime lens? (85mm for APS-C.) –  mattdm Jan 18 '12 at 19:56
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common usage of the word "prime" refers to a fixed focal length lens, i.e. not a zoom lens. –  MikeW Jan 19 '12 at 4:40

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