While fixed-focal-length supertelephoto lenses (400mm and up) are popular with wildlife pros and sports shooters, there's something to be said for the more than 50 telephoto zoom lenses on the market. Tele-zooms provide long focal lengths and add framing flexibility—quite handy when you can't easily move toward or away from a subject in the field. Zooms also mean fewer lenses to lug into the field and fewer lens changes (a plus for D-SLR users who want to keep their image sensors dust-free). And for those on tight budgets, zooms that go to 300mm can be had for far less than fixed-focal-length 300mm lenses.
Like fixed-focal-length lenses, tele-zooms come in a wide variety of price ranges. You can get an entry-level model for less than $200 or pay more than $5,000 for a top pro optic. The high-end pro lenses offer many benefits over entry-level lenses, including faster maximum apertures, better image quality, faster autofocus performance, better materials and more-rugged construction—after all, that's why they cost so much. If you're on a tight budget, the major-brand, entry-level telephoto zooms deliver good image quality and let you get started in telephoto photography. But remember that it's the lens that makes the picture. The best pro camera body with a mediocre lens can't deliver better than mediocre image quality, while an entry-level SLR with a good lens can turn out very good image quality, even if it won't shoot 8 fps or withstand the elements like a pro model.