Zoom lenses are a popular compromise for many users. The advantages are obviousyou get the equivalent of many different focal lengths all in one compact package. Most "kit lenses," or those that come with a digital SLR, are zoom lenses that cover the wide-angle to telephoto range (Figure 3.3)
Figure 3.3. A typical kit lens will give you the equivalent of a 28-90 mm zoom. These lenses are usually not the best optical lenses, but they are a good starting point if you're just getting into dSLR photography.
The most common zooms cover the 35mm film equivalent of 28mm to 90mm focal-length lenses, and often have a variable maximum aperture, which keeps the cost and size of the lens down. An example of this is the standard lens included with the Canon Digital Rebel. This zoom lens has a focal length that ranges from 18-55mm with a maximum aperture that varies from f/3.5-5.6. In lay terms, the lens has a maximum aperture of f/3.5 when set to 18mm, and f/5.6 when zoomed out to 55mm. Because the Digital Rebel's sensor is smaller than the traditional 24mm x 36mm film frame, it has a magnification factor of 1.6x. Therefore this lens would be equivalent to a 28-90mm lens on a standard 35mm camera. The focal length is different, but the aperture doesn't change. A professional-quality zoom lens with a constant aperture setting that doesn't change when the focal length is increased, such as the Canon 24-70 f/2.8 shown in Figure 3.4, is larger, heavier, and more expensive than a comparable lens with variable aperture. However, it does have an advantage in speed and in depth of field control. At the longer focal lengths you still have the wider, faster aperture setting giving you faster shutter speeds, and the narrower depth of field is often an advantage with composition (more on this in a bit).
Figure 3.4. This image to the right was captured with a telephoto zoom lens. Because the boat was moving so fast, being able to change focal lengths allowed me to compose the image quickly. Photo taken with a Canon 1D Mk II, 70-200 with 2x converter at 1/640 and f/8.
Zoom lenses are great when you can't move closer to your subject, such as when shooting wildlife or distant objects. As an example, the hydroplane image in Figure 3.5 would be very difficult to capture with a fixed-length lens. I was standing on the edge of the water and the boat was moving at well over 100 miles per hour, which required flexibility in framing that only a zoom lens would provide.
Figure 3.5. Changing the focal length while capturing the image lets you create some interesting effects. Photo taken with a Canon 1Ds Mk II, 24-70 at 1/2 second and f/22.