Telephoto lenses
are great for shooting wildlife.
Telephoto lenses, available in every price range, use optics to bring your photographic subject closer. Any lens with a focal length greater than 50mm is technically a telephoto lens.
Prime vs. Zoom
Shooting with a telephoto lens.
A telephoto for a digital single-lens reflex may be a prime or zoom lens. A prime telephoto lens has a fixed focal length (as in 100mm), while a zoom telephoto has a variable focal length (as in 70-200mm). According to Darren Rowse, of the Digital Photography School, "Zoom lenses are the most popular DSLR lenses at present and come in a range of configurations and levels of quality."
Weight
Using a super-telephoto lens.
Telephoto lenses can exceed focal lengths of 600mm; however, a longer focal length results in a heavier lens. When using a super-telephoto (300mm plus), a tripod is often needed to prevent camera shake.
Lens Speed
A fast lens is more versatile in low-light settings.
The speed of a telephoto lens is determined by its largest f-stop or lens opening. By recording more light, a larger f-stop (as in f2.8) allows the photographer greater flexibility in low-light situations.
Price
Telephoto lenses can be expensive
Better optics, a longer focal length and a faster lens speed are three factors that will increase the price of a telephoto lens. CNET is a great site to review available lens choices.
Conversion Factor
APS-C vs. full-frame.
Digital single-lens reflexes are available in both APS-C and full-frame formats. An APS-C camera has a smaller sensor, resulting in lens focal length conversion factor. For example, the conversion factor for Canon APS-C cameras is 1.6x--meaning that a 200mm telephoto would have the equivalent focal length of 320mm.
Tags: focal length, Telephoto lenses, telephoto lens, APS-C full-frame, conversion factor