Zoom lenses provide a whole range of focal lengths in a single package and are very popular for that reason. But they also offer a great way to acquire long focal lengths on the cheap: While the lowest-priced 300mm fixed-focal-length lenses cost around $1,400, you can pick up a good 70-300mm zoom from the same camera maker for less than half that (avoid the really low-priced telezooms). You're going to lose a stop of lens speed, and some AF and optical performance, but the $400-$700 70-300s are good performers and great choices for the budget-limited wildlife shooter. Zooms prove an even better deal at longer focal lengths: While fixed-focal-length 500mm superteles start at around $5,000, Sigma and Tamron offer 150-500mm and 200-500mm zooms, respectively, for about one-fifth that price. Again, you'll give up some lens speed and AF and optical performance, but you'll still get very capable lenses—and focal lengths you otherwise couldn't enjoy.
The 70-200mm Zooms
Many outdoor photographers love the 70-200mm zooms (available in fast ƒ/2.8 and—from Canon for EOS DSLRs—ƒ/4 varieties), both for landscapes and for wildlife subjects they can approach fairly closely. We didn't include them here because they aren't really supertelephotos (although you can add a 2.0x teleconverter to a 70-200mm ƒ/2.8 and get a versatile 140-400mm ƒ/5.6).
Teleconverters are small tubes that fit between the camera body and a telephoto lens. They contain one or more elements and increase the focal length of the lens to which they're attached by 1.4x, 1.7x, 2.0x and even 3.0x. Advantages include cost (a $100-$500 converter is a lot cheaper than a supertele lens), compact size and no change in minimum focusing distance: Attach a 2.0x converter to a 300mm ƒ/4 lens that focuses down to 4.9 feet, and you get a 600mm ƒ/8 lens that focuses down to 4.9 feet. The drawbacks are a slight loss of image quality, slowed (or nonexistent) AF performance and (as hinted in the preceding sentence) a loss of light: one stop for a 1.4x, 1.5 stops for a 1.7x and two full stops for a 2.0x. Most DSLRs require ƒ/5.6 for autofocusing to work, so adding a converter to a slower lens, like a 70-300mm ƒ/4-5.6 zoom, means having to focus manually—and ƒ/8 with a 1.4x converter and ƒ/11 with a 2.0x provide a very dark viewfinder image for manual focusing. That aside, a 300mm ƒ/4 lens and 1.4x converter give you a 420mm ƒ/5.6 lens that focuses down to around five feet, great for everything from birds in flight to flowers and butterflies (the only major-brand 400mm ƒ/5.6 fixed-focal-length lens has a minimum focusing distance of 11.5 feet).