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There are many discussions of differences between prime, zoom and macro lenses. I have not heard about prime lenses until recently.

Could someone explain what exactly a prime lens is?

Is it just that the focal length is fixed?

And if so what are the advantages of having the focal length fixed?

Also why the name 'prime'?

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I thought it was because of the 'Prime' cost! –  Chuck Conway Aug 3 '10 at 21:20
@Chuck: Craigslist is your friend! I've bought a few prime lenses, used, on there and saved a bundle. Upside to being a Pentax shooter is all these people selling their parent's old gear with no idea what gems they sometimes include. –  John Cavan Aug 3 '10 at 21:31
@Chuck Canon's cheapest lens is a prime... –  Rowland Shaw Aug 4 '10 at 11:51
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6 Answers 6

up vote 32 down vote accepted

In practice, you're correct and it means that it's a fixed focal length lens.

Historically, the term "prime" derives from cine work, where it had a more literal meaning of the primary lens of a multi-lens system.

The concrete advantages are typically that they are much more compact, and have a much higher quality/cost ratio, as they're easier to engineer to a high degree of correction. There is no such thing as an f/1.4 zoom lens, for instance, but f/1.4 primes exist in several focal lengths.

Zoom lenses are catching up on the quality front, a particular example being the Nikkor 14-24, which is reputedly as high-quality as the primes of similar focal lengths (and the Nikkor 24mm is considered a classic design).

The more subjective pros/cons are – obviously – a lot more subtle, and vary from person to person. For example, some people find that primes let them focus more immediately on the composition of the image and/or their surroundings (i.e., it's one less camera setting to think about), while others prefer the ability of a zoom to quickly change the field of view, producing extremely different compositions in quick succession. The same person might like both in different situations. I think the only fair answer to this aspect of the different approaches is to try using a prime regularly and see how you get on with it.

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in autofocus systems another advantage of prime lenses (actually derived from the lower weight) is that they tend to focus much more rapidly. –  jwenting Mar 28 '11 at 12:43
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It's a fixed focal length lens. The primary advantage is simpler optics which, in turn, usually leads to wider apertures and sharper images. Generally, the more glass you throw in front of the sensor to accomodate zooms, the more correction you have to do, with glass, which then leads to smaller apertures and optical degradation.

Not sure on the name 'prime' specifically, but I'm sure there's a good historical reason for it.

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The most common meaning presently is "fixed focal length." (A less common meaning is "the primary lens in a multiple lens set-up" such as in a Twin Lens Reflex camera.) I've no idea why "prime."

Some people seem to use it more specifically for a fixed focal length lens with "normal" perspective. In thus use, for a 35mm camera, a 50mm or 55mm lens would be prime, and a 28mm or 135mm would not.

All else being equal, prime lenses have better optical quality than do zoom lenses. They have fewer moving parts, fewer elements, and are designed to perform best at their single focal length. Zoom lens design involves trade offs and compromises such that a zoom will not perform optimally at all of its range of focal lengths, and rarely if ever, as well as a prime for any given length in its range.

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A prime lens is merely one with a fixed focal length. They are considered to be of a higher quality, and are capable of wider apertures as the mechanics tto cope with zooming don't need to be incorporated.

There is an extended discussion on the pros and cons in another question here on the site

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A couple of other advantages of primes that can be important depending on the situation:

Lens hoods for zoom lenses have to be designed for the widest focal length so they don't cause vignetting when you zoom out. Prime lens hoods are designed to be optimal for one particular focal length and thus provide slightly better protection against flare compared to a zoom at the telephoto end.

Prime lenses are easier to calibrate (e.g. to account for barrel distortion, CA, or light falloff) as these factors vary throughout the range of a zoom lens thus you must calibrate for many different focal lengths.

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A prime lens is a lens which does not zoom. That is, as you say, it has a fixed focal length.

All lens design is a matter of balancing competing factors, including various optical properties which affect image quality in different ways, as well as size, weight, and cost. Zoom introduces a significant complication into this compromise: not only is it complex itself, but also, it means that the other compromises can't be made to fit just one focal length. That means that zoom lenses are usually more expensive or have lower image quality (or, sometimes, both at once). Almost always, maximum aperture is the first to go, with f/2.8 being the normal (but not universal) limit for a "fast" zoom, while many primes are f/2.4, f/2, f/1.8, or lower.

Additionally, some people find that they enjoy working with prime lenses. I won't elaborate too much on this here, but see Would a fixed or zoom telephoto lens be better for learning? and particularly How do I compose photos with prime lenses?

As for the origin of the term: it's clearly a retronym — a new word made up to cover something that didn't need its own term until something else came along. (Like, "acoustic guitar" or "land-line telephone".) In this case, that's zoom lenses: before they were popular, a lens with a fixed focal length was just a lens. If you look at this search for prime lens in books before 1960, you'll notice that the term "prime lens" is used in opposition to a secondary adapter lens which attaches in front. This makes perfect sense, and in fact we still call that kind of adapter lens a "secondary lens".

Most of these results are in cinema or industrial photography. If you extend the search through the 1960s, you'll see the same use as a distinction for "the main lens that you might attach an adapter to", and increasingly in still photography. One particularly interesting snippet (full book not online) describes a zoom lens design like this:

In its simplest form the zoom assembly may be thought of as a combination of an afocal attachment of variable power placed in front of the remainder of the components composing a prime lens.

And there are similar quotes in other books from around the same time.

This very well may be the bridge by which the term entered the popular vocabulary — by 1978, there are articles talking about "prime, fixed-focal-length lenses" as opposed to zoom lenses. By the early 1980s, the usage we are familiar with now is commonplace.

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