In photography, a "prime lens" is one that does not zoom; it has a fixed focal length. A "normal lens" is one that gives a horizontal picture angle of 45 degrees. Lenses with shorter focal lengths are called "wide-angle"; lenses with longer focal lengths are called "telephoto". Historically, normal lenses have been popular for photography; some photographers never used anything else.
Tradition (& marketing) suggest that a 50 mm focal length lens is the normal prime for single-lens reflex cameras that take 35 mm film or have "full frame" digital sensors (i.e., with a detector sized 24 mm x 36 mm, same as a frame of 35 mm film).
But.
In fact, on 35 mm film, a 50 mm lens gives a picture angle of 39.6 degrees, clearly less than 45 degrees, and therefore inadequate. Simply sub-normal.
A true normal lens for 35 mm film (or a full-frame digital sensor) would be one with a 43.33 mm focal length. While no one makes such a lens, Nikon does make an autofocusing 35 mm f/2.0 lens, which gives a picture angle of 54.4 degrees. Yes, that's wide. Not super-wide. But at least it is at least normal.
So, if you're shooting full-frame, and you want to be at least normal, don't respect conventional wisdom (& marketing) by choosing a 50 mm lens. Because fifty ain't normal, no matter what they say.
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