Lens Basics Lenses don't have fun names, but rather are labeled like this: 18-55mm f/3.5-5.6 . That designation tells you a lot, but doesn't mean much if you don't know a few terms. IS = image stabilization - an electro mechanical system in the lens that counteracts camera shake. USM = Ultrasonic Motor - refers to the focusing motor in the lens. USM lenses focus faster. Zoom Lenses vs. Prime Lenses Zooming Lens A zoom lens does what you might expect: it zooms in and out, allowing you to magnify your subject. A prime lens can't zoom. If you want to move closer or farther away with a prime, you move your legs rather than turning a zoom ring on the barrel (i.e. the body) of the lens. Why would anyone prefer a prime to a zoom? Primes are often cheaper, offer sharper results, have wider apertures (which means they can take in more light, thus performing better in low light situations), and generally handle their specific function
A digital single-lens reflex camera (also called a digital SLR or DSLR ) is a digital camera that combines the optics and the mechanisms of a single-lens reflex camera with a digital imaging sensor , as opposed to photographic film . The reflex design scheme is the primary difference between a DSLR and other digital cameras. In the reflex design, light travels through the lens, then to a mirror that alternates to send the image to either the viewfinder or the image sensor. The traditional alternative would be to have a viewfinder with its own lens, hence the term "single lens" for this design. By using only one lens, the viewfinder of a DSLR presents an image that will not differ substantially from what is captured by the camera's sensor . A DSLR differs from non-reflex single-lens digital cameras in that the viewfinder presents a direct optical view through the lens, rather than being captured by the camera's image sensor and displayed by