Prime lenses can be faster (both in aperture and autofocusing speed), sharper, and much lighter in weight than zoom lenses. If you’re looking for the ultimate in image quality, a good prime lens or two may be exactly what you want. In all cases in the following list, when referring to recommended focal lengths, I use the actual focal length, but I assume that your camera has about a 1.5X crop factor. That number works for the majority of consumer-priced dSLRs on the market, and isn’t too far off for those cameras that have 1.3X to 1.6X crop factors. Here’s a list of some common applications and the prime lenses that you might find especially suitable for them: 


Architecture: Wide-angle lenses in the 12mm-to-28mm range (depending on how much of the wide view your system’s crop factor clips off). Most of the time, you’re shooting with a relatively small aperture to gain depth- of-field, so an f/2 or f/1.4 maximum aperture isn’t as important, even when shooting indoors under low light levels. A tripod serves you better. If you shoot a great deal of architecture, you can purchase a perspective control lens that compensates for the falling-back effect that results when you tilt the camera back to capture the upper reaches of a tall structure.


Indoor sports: You want a lens that has a moderately wide angle of roughly 30mm focal length for indoor sports, such as basketball or volleyball, as well as long lenses for shooting from up in the stands. You can use large apertures for available-light sports, but only if you can focus or pre-focus carefully on a single subject or two because depth-of-field is limited, as you can see in Figure 6-4. Autofocus may not accurately focus exactly where you need to when using a very large aperture. 


Outdoor sports: Longer lenses are useful, and primes in telephoto focal lengths often offer an excellent combination of reach and speed. You’ll find 100mm-to-200mm lenses, or longer, great for capturing football, baseball, soccer, and other outdoor sports. Apertures of f/2.8 to f/4 let you use faster shutter speeds to freeze the action (again, assuming that you or your camera’s autofocus system make the most of the available depth-of-field). 


Portraits: If you’re taking head-and-shoulders shots, a 50mm-to-70mm prime lens offers the same flattering perspective as the traditional 75mm-to-105mm portrait lenses used with 35mm full-frame cameras (with a 1.5X crop factor on the dSLR). Such prime lenses usually come with f/1.8 or faster maximum apertures, which you can use to limit depth-of-field when you want to concentrate on your portrait subject’s face. Longer focal lengths can produce a flattening effect on the features, and shorter focal lengths can make ears look too small while enlarging noses if you use those focal lengths up close. 


Macro photography: Prime lenses, such as the ever-popular 50mm f/1.8 or f/1.4, although not intended for close-up work, can still do a great job. You may have to purchase extension tubes or other accessories to focus close enough to fill the frame with very small subjects. 


Landscapes: Wide-angle primes (12mm–24mm) can give you landscape photos that are sharp enough to blow up to 16 x 20 inches or larger, making framed pictures to grace your walls. 


Of course, the only problem with using prime lenses is that you must be willing to swap lenses whenever you decide to shoot something else or need a different perspective that you can’t get by stepping closer or farther away. Digital SLRs have one additional consideration: If you’re working in a dusty environment, you may not want to change lenses a lot because each time you take off a lens, you let some dirt invade the camera body and, possibly, end up on the sensor.