If there’s one technical aspect of photography that causes more confusion for beginners than any other, it’s probably aperture. I feel like I explain it a lot, and just this week a friend was asking about a lens purchase and the subject came up again; I thought it was about time to write a quick post on the subject.

The aperture affects how much light is able to reach the camera’s sensor. There’s a bladed diaphragm inside the lens that expands and contracts, depending on how you’re using the camera and what settings you’ve chosen, to allow more or less light in. The aperture is expressed as “f/2.8″ or “f/4″.

What do those numbers mean? The smaller the number, the larger the aperture (meaning more light gets in). That may seem confusing until you think of the expression as a fraction. 1/2.8 is greater than 1/4 is greater than 1/8. Lenses specifications are described in the maximum aperture the lens can achieve — e.g., the Nikon 50mm AF-S f/1.4 can “open up” to f/1.4, which is very wide. Minimum apertures are rarely a concern; that 50mm Nikon can “stop down” to f/16. Cheaper zoom lenses, like the kit lens that came with my Nikon D40, sometimes have variable minimum apertures. For example, my 18-55mm lens is rated as “f/3.5-5.6″. That means that at 18mm it can open up to f/3.5, and as you extend through the zoom range up to 55mm you eventually can’t open it any wider than f/5.6.
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