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" :-)
Last edited by Phong; 12-05-2005 at 02:36 PM. Click to view previous post history.