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 ...