There’s a very useful article on MacWorld about how to buy a lens. It goes into some depth about the very basics of how a lens works: its focal length and aperture. What I’ve been looking to understand for a while is the meaning of the f-stop values. I’ve read a few explanations but this time it really sank in.
The aperture is usually referred to in terms of f-stops, and is represented by a number such as f/2.8. The smaller the number, the larger the opening, and the more light it allows into the camera. Because it collects more light, a lens that opens to a wide aperture lets you maintain faster shutter speeds in low light-this can be essential for obtaining sharp images from a handheld camera. Wider apertures also provide more creative control by giving you the option to throw backgrounds out of focus.
Have a read if you’re interested in digital SLR lenses, but don’t have much experience with photography. The lens recommendations and comments are quite interesting.