The aperture is the opening in the lens that changes in size to let in more or less light. Numbers called f-stops indicate how large or small the opening is. When dealing with f-stops, keep in mind that smaller numbers relate to larger lens openings. Larger numbers refer to smaller openings.

As you move up the f-stop ladder, each number lets in half as much light as the previous one. For example, f/8 lets in half as much light as f/5.6.

FIGURE 6-2 This is what the aperture openings look like on a 50mm lens. On the top row (left to right) are f/1.4, f/2, f/2.8, and f/4. On the bottom row (left to right) are f/5.6, f/8, f/11, and f/16.

All SLRs have identical f-stop numbers, but the settings you'll actually be able to use will vary depending on the aperture range of the lens you're using. For example, the dial on your camera body may go down to f/2, but you might be using a lens that only opens down to f/3.5. In this case, although your camera can handle a wider aperture, your lens can't, and you'll be limited to what the lens can do. Many zooms change f-stop as you zoom — f/2.8 when at wide angle, but f/3.5 when zoomed to telephoto. There are also zooms that have a constant aperture at any zoom.

F-stops can be confusing. For example, it may seem that an f-stop of 4 (f/4) would be double the light of f/8. There's a long mathematical explanation for why it's not, but it's perfectly acceptable — and easier — not to go into it. Just accept the settings for what they are, and understand how they relate to each other. This may seem a bit confusing now, but after a while it will become second nature.