A prime lens is one that has a single focal length. A zoom lens is one that has a range of focal lengths. The each have their proper place in the camera bag and on the camera. But the prime has always been, and will continue to be, a favorite among seasoned photographers using interchangeable-lens cameras.
I picked up my first prime lens nearly a year ago (105mm f/2.8 macro), and over the course of the year I've grown to consider that lens as my favorite. In the last few months, I've hardly taken the lens off my camera body. But then I picked up another prime lens (50mm f/1.4) and I'm absolutely in love with these things. Here's why:
2. LOWER COST
For the same quality, prime lenses cost less than zoom lenses. They contain fewer elements, less moving parts, and their design is simpler. For the same cost, you can pick up a half-decent zoom lens or you can pick up an outstanding prime lens.
3. FEWER LENS DISTORTIONS
Zooms are designed to work well at most focal lengths in their range, but the all display some type of lens distortion at some point — usually at the extremes. Primes, on the other hand, are designed to work great at a single focal length. The distortions have been minimized by design.
5. SHARPER IMAGES
Similar to the argument for lens distortions, primes have been optimized for sharpness and clarity while zooms must sacrifice these things in order to offer up the convenience of multiple focal lengths.
7. BETTER DOF CONTROL
Again, for the same price point, prime lenses are capable of a wider array of f-numbers. They're faster, and they offer more options at the low end of the f-number scale.
11. NICER LOOKING BOKEH
Generally, as you lower your f-number your bokeh becomes more apparent. Primes are notorious for producing crazy bokeh on specular highlights when shot wide open. Primes will also generally have better and/or more aperture blades, thus giving you a better bokeh.
13. LOW LIGHT CAPABILITIES
If you've never shot with a f/1.4 (or faster) lens, you have no idea what you're missing. Indoor shots — no flash, no problem. Concerts — fast lenses are a must.
17. THEY MAKE YOU THINK
I suppose my favorite thing about prime lenses is the fact that you have to use your head. Composition becomes a thinking game. You have to move your feet to get that shot you had in mind, so you really start to evaluate what's important in the scene. Fast primes also make you think a little harder about your f-number. The DOF can be extremely shallow; sometimes too shallow to produce an effective shot. Not only that, but on bright sunny days, you actually can't use the lens wide open without an ND filter because you'll let in too much light and max out your shutter speed.
So if you don't have a good prime lens, you're really missing out. Zooms are fine, and they have their place, but a prime will open your eyes to a whole new level of photography.