Aperture and shutter speed settings control the amount of light hitting your sensor. The shutter speed does this by limiting the amount of time the sensor sees the scene. The aperture controls the light by changing the size of the opening (the aperture) that the light comes through.

The aperture of your lens opens and closes, letting in more or less light depending on the setting. The more light coming in through the aperture (the larger the aperture setting), the faster your shutter speed will be. Consequently, the less light (the smaller the aperture setting), the longer your shutter speed will need to be.

The Aperture Effect

Aperture and shutter speed settings have a direct effect on the look of your photographs. While your shutter speed controls time and motion, the aperture setting controls your depth of field (how much of your scene, front to back, is in focus). Where a portrait with a blurred background is done with a large aperture (say f2.8), a landscape image might be shot at a small aperture (like f16) to ensure sharpness throughout the frame.

Different aperture settings produce images with differing looks. The larger aperture settings give you a shallower depth of field, which results in blurred background and foreground. Smaller apertures enlarge the lens's depth of field and keep more of the foreground and background in focus.

Exposure

Apertures are divided into stops. Going down the scale from the smallest aperture to the largest, each stop lets in twice as much light as the one before it. For example, the aperture f4 gives you twice the light as the aperture f5.6. Going up the scale, each stop lets in half as much light as the one before it. (On most modern cameras, these stops are broken down into half-stops or third-stops, allowing for greater exposure accuracy.)

In every photo situation, you have a variety of aperture settings that will result in a perfect exposure. You're never locked into one speed. Because as you let in more light with larger apertures, you can lower the amount of light coming through the lens with a faster shutter speed. So if your meter tells you that the proper exposure is f4 @ 1/250th of a second, you can also shoot at f5.6 @ 1/125th, f8 @ 1/60th, f11 @ 1/30th, f16 @ 1/15, f22 @ 1/8th, on and on, and still get a correct exposure. As you let in less light via the aperture, you are letting in more light via the shutter speed.

Apertures and Lenses

Like the aperture setting, the focal length of the lens will directly affect your depth of field. The lower the focal length, the more depth of field. Wide angle lenses have a wide depth of field while telephoto lenses have a shallow depth of field. The longer a telephoto lens is the shallower its depth of field will be.

You can use this effect to great results. Telephotos are great lenses for isolating a subject with selective focus and shallow depth of field. Using a telephoto at larger apertures enhances this effect.

Think of a wide angle as the lens that keeps everything crisp. Using a smaller aperture on a wide angle lens will enhance that effect.