For a while now I've been using my zoom lens a lot, but after getting a Panasonic GF1 with 20mm f/1.7 lens I realized how much I miss using prime lenses. After getting and using a 28mm for my SLR I'm reminded how versatile prime lenses can be. Here are my top reasons for liking to pull out a prime lens over my zoom:
1. Rejoice - no choice! There is something freeing about having only one focal length. If you have some room to move about you can accomplish a lot of selection on the background and choose to emphasize your subject. If you use it enough you get used to the field of view and start seeing compositions that work for that lens. On a zoom you are much more tempted to zoom first rather than move around a bit.
2. Minimum focus distance. My zoom can only focus down to 0.45m, while my 28mm lens can focus to 0.25m. This is the difference to being able to grab a shot of the kids when they are playing on me (youngest is 16 months old) versus having to back up a bit. With the zoom I can get more magnification by zooming to the tele setting - but then it fills too much of the frame. With prime lenses you can usually focus closer allowing you to emphasize the subject - especially handy for a wider lens.
3. Light weight. No contest here - most primes are lighter than a quality counterpart zoom. 28mm lens 310g, zoom 670g. This is not a rule, of course, as there are light zooms and heavy primes.
4. Aperture. This is a big one - there just isn't a zoom lens available with an f/1.8 aperture*. This helps focus speed and accuracy, helps to get blurred backgrounds, and makes the viewfinder brighter. Sure it may not be 100% sharp wide open, but stopped down to f/2 to f/2.8 it is as good or better than most zooms at f/5.6.
5. Fixed size. I like having a lens that doesn't extend, rotate, etc. I don't have to worry about it getting jammed or extending in a bag, etc. Keep it simple!
6. Wildcard. This last point is one that I think about a lot, but doesn't show up on any reviews I've ever read. You can operate a prime lens with one hand! When I'm chasing kids around I can't overstate how handy that is. With the zoom it is always at the wrong focal length. With my hand strap and a prime lens I can change aperture, focus, adjust exposure compensation and still carry a baby in the other arm :)
7. Cost. A lot of prime lenses are very cheap for the quality. They are great bang for the buck for stock. A cheap 50mm f/1.8 can take you a long way for $129.
Naturally my next article will be why zooms rock :)
*Olympus does make a very nice f/2 zoom - doesn't work on my camera though, and a little pricey :)