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To
get the most out of your D-SLR system, you’ll want to choose a variety
of lenses that offer the focal lengths, speed and features that match
well with your favorite subjects. The key to selecting the right lenses
is in understanding their capabilities and limitations and the types of
photography and situations to which each is best suited.
First, we’ll examine the basics of lens design and performance, then
we’ll explore the general categories of lenses that address particular
photographic needs.
 FOCAL LENGTH, PRIME AND ZOOM
The first and most obvious lens feature is focal length. This
millimeter measurement ultimately represents the area of coverage and
magnification that a lens provides. A short focal length (like 15mm)
provides a wider angle of view without much magnification and a long
focal length (like 200mm) provides much more telephoto enlarging power.
In the 35mm film format, wide-angle lenses are usually considered
anything less than 40mm. At the opposite end of the spectrum are
telephoto lenses. Again, in 35mm terms, these lenses have a focal
length of 60mm or more. Lenses between 60mm and 100mm are often called
short or medium telephoto lenses. Between the wide-angles and
telephotos are “normal” lenses. With a range of 40mm to 60mm in 35mm
film terms, these lenses are designed to approximate the view of the
human eye. When any of these lenses has a fixed focal length, it’s
called a prime lens. In lieu of primes, some photographers opt for
zooms—a single lens that offers a range of focal lengths in a single
package.
Unlike 35mm cameras, digital cameras vary in sensor
size, and this directly impacts the angle of view a particular focal
length provides. D-SLRs that are referred to as “full-frame” have
sensors that match the size of the 35mm frame. When you mount a 35mm
lens to a D-SLR with a full-frame sensor, that lens behaves as
described above.
However, most consumer-class D-SLRs have
sensors that are smaller than a 35mm frame, and when paired with a 35mm
lens, create a telephoto effect, usually around 1.5x magnification.
That means that a 24mm lens will actually provide an angle of view
equivalent to a 36mm lens. Keep this in mind when shopping for lenses
if you don’t have a full-frame camera. This fact can be a boon for
telephoto work, but it also means you need an even wider lens to do
wide-angle photography.
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Buyer's Guides 
