Automatic cameras, on the other hand, adjust the shutter speed or lens opening (or both) automatically, after determining an optimum exposure setting. Automatic cameras equipped to handle film with DX-encoding designations even set themselves for the speed of the film you're using.

Whether your camera uses a built-in meter to guide you in setting aperture and shutter speed or sets them itself, you should understand the basic premise behind shutter speed and aperture to gain greater control over image quality. The shutter speed controls the length of time the film is exposed to light. Shutter speeds are indicated by the numbers 1, 2, 4, 8,15, 30, 60, 125, 250, 500, 1000, and 2000. The speeds may be marked on a dial or shown on a liquid crystal display (LCD) panel atop the camera or in the viewfinder. Your camera may not have all of these speeds. The numbers represent fractions of a second (except 1 second) and mean 1/2, 1/4, 1/8, 1/15 second, and so on. You can use the B setting to make time exposures-the shutter will stay open as long as you press the shutter release. For more precise control of time exposures, some advanced cameras allow you to set electronically timed shutter speeds of up to several minutes.

The lens openings on cameras are indicated by f-numbers The larger the f-number, the smaller the lens opening. Each smaller (size) lens opening marked on the lens opening scale lets in one-half the amount of light as the preceding opening. If you change from a small lens opening to the next larger one, the lens will let in twice as much light. On some camera lenses, the maximum lens opening may not let in twice as much light as the next smaller opening. You can also set the lens opening between the marked settings on the lens for finer changes in exposure. The illustrations below the picture show how the lens opening varies with the f-number setting.

Changing from one shutter speed to a speed that is twice as fast, for example 1/60 to 1/125 second, allows the light to strike the film for half as long; therefore half as much light reaches the film. Changing to a shutter speed that holds the shutter open twice as long, for example 1/60 to 1/30 second, lets twice as much light strike the film.

The size of the lens opening on your camera is the other factor that controls the amount of light that reaches the film. The different sizes of lens openings are indicated by f-numbers. These numbers form a series, such as 1.4, 2, 2.8, 5.6, 8, 11, 16, and 22, marked on the camera lens or shown on an LCD panel. The smallest f- number refers to the biggest opening. The largest f-number is the smallest lens opening.

When you change from one lens opening to the nearest number, you're adjusting the lens by 1 stop. If you move the setting to the next larger one, for example f/11 to f/8, the area of the opening is doubled, so you expose the film to twice as much light. Changing from one lens opening to the next smaller one, for example f/11 to f/16, cuts the light by half.

Automatic-exposure cameras dominate the camera market. Electronic sensors and microprocessors have not only taken the guesswork out of correct exposure but the labor as well. The camera sets the shutter speed and aperture the moment you press the shutter release. Cameras that measure the light reflecting off of the film itself can even adjust these settings as the exposure is occurring.

Using the settings going up each column exposes the film to more light. As the setting in one column is increased, the setting in the other column must be decreased to maintain the same exposure for the lighting conditions in the scene you want to photograph. For example, suppose the correct exposure setting is 1/125 second at f/8. To help stop fast action, you can change the shutter speed to 1/250 second, but you must also change the lens opening to f/5.6 in order to maintain the same exposure. If stopping fast action is not required but you want to increase the depth of field, you could use 1/60 second at f/11 which also maintains the same exposure.