When you take a picture, you "expose" a film or sensor to light. The two parts which work together to control your exposure are the APERTURE and SHUTTER. Some "Point and Shoot" cameras select these automatically, but more expensive digital cameras enable you to set these manually, or to "program" them for certain shooting conditions.

The aperture is an opening that changes in size to admit more or less light (similar to the iris of an eye). The numbers on the aperture control are called F-stops and referred to as F16, F11, F8, and so on.

Here's how it works:

The larger the F-stop number, the smaller the opening.

Each number higher lets in half as much light as one number lower.

For example, F5.6 admits twice as much light as F8, while F11 lets in only half as much.

The aperture doesn't work alone, however. The shutter speed is responsible for exposure, too. It controls the amount of time light is allowed to reach the film or sensor.

The shutter is a device that opens and closes at varying speeds to determine the amount of time the light entering the aperture is allowed to reach the film or sensor.

Shutter speed is measured in fractions of a second. 125 means 1/125 of a second, 60 means 1/60. Typical shutter speeds range from 1 second to 1/1000. A shutter speed setting for a bright, sunny day - using an aperture of F11 - might be 1/125 second. A cloudy day might use 1/60 second with the same aperture, exposing the film or sensor to light for a longer period of time.

The settings for a good exposure are determined by a light meter. (Most 35mm cameras have a built-in light meter that shows you the appropriate settings, or automatically controls them.)

Aperture and shutter settings work together. Because the shutter (like the aperture) approximately halves or doubles the light reaching the film or sensor with each change in setting, a number of different combinations of settings can result in the same exposure.

Any of the combinations shown above would result in approximately the same exposure.

If all the settings result in the same exposure, why would you want to use F5.6 at 1/125 instead of F11 at 1/30? Two good reasons: By selecting the right combination for the situation you can control depth of field and motion blur.

Before you explore more about digital cameras, computer software, and photographic techniques; focus on lighting situations and the various fields or kinds of photography.