What is the definition of a “prime lens”?

Question

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'?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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

Answer 5

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.