My cabal of photogs has no idea what a "Prime" is, and neither do I. Yet, it seems to be a popular term on the Net. Could someone please define it for me. So far We've gathered it's one of the following: 1) a fixed focal length lens 2) the "best" lens of that fixed focal length for whatever camera. 3) really really nice glass. 4) lenses that come in focal lengths of 23mm 59mm 103mm or 151mm my money is on #4

Is it possible they mean "primary" in which case I would think "normal" lens, hence, 50 mm for a 35 mm camera, 210mm for a 4x5, etc.?

Well in all my classes I have always been taught that a "prime" lens is a single focal length lens, with out zoom capibilities, the actual focal length does not matter...

It's a single focal length lens. Actually there are variable focal length primes but they're made for 35mm movie cameras. "Vari-primes" by zeiss come in a couple sizes and are much smaller than full range zooms but slightly larger than standard prime lenses of similar focal lengths.

Now, why they had to pick up the term "prime" among the thousands of better descriptive ones, is another question...

The phrase "prime lens" is generally used to mean "lens with fixed focal length" today. Long before zoom lenses came around, it used to mean the primary lens (set of lens elements or lenses, to be precise) in a complex optical system, in the sense that the system was designed for optimal performance with that set. You can add "supplementary lens" elements to the "prime lens" to change the focal length of the prime to get more mileage out of it. Supplementary lenses are calibrated in diopters; negative diopters increase the focal length of the resulting system; positive diopters reduce it. In a sense, you could think of the 240mm lens in a 240mm/420mm convertible as the "prime", which can then be converted not by adding, but by removing a lens from the system. With the advent of zoom lenses with their ability to change focal length continuously through the zoom range, "prime lens" lost its original meaning, as focal lengths change not by removing or adding elements (a discrete process) but by changing the distance between lenses (a continuous process), and came to mean "lens with fixed focal length". "Prime lenses" have nothing to do with "prime numbers" Cheers, - Phong

Phong, I knew that a prime lens was a lens with fixed focal length. However, I was totally unaware of the history behind the term "prime lens." Thanks for a very informative posting.