Online Tutorials: Aperture
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The aperture controls the amount of light that passes through the camera's lens. If you look at a standard lens, the aperture is simply the hole that light is allowed to pass through. It is commonly controlled by adjusting a dial on the camera body, automatically by the camera or on older lenses by rotating the aperture ring on the lens.
The smaller the f-number on your lens (f2, f2.8 etc.), the wider the aperture on the lens (i.e. more light can pass through the lens in a given time) and the higher the f-number (f11, f16, f22 etc.) the narrower the aperture is, meaning that less light passes through in a given time.
The f-number is calibrated to allow exactly half as much light through as the previous setting and twice as much light through as the next setting (in a set time of course). For example, a lens set at f8 will allow twice as much light through as one set at f11 but only half as much as one set as f5.6.
The aperture not only controls the amount of light entering your camera, it controls how long the shutter has to be open to achieve a corect exposure. A wider aperture needs less time than a smaller aperture. It also controls the amount of the image that is in focus, otherwise known as depth of field.
These images show the different apertures sizes on a particular lens. stepping down from F22 to F8 and then F3.5.
The aperture can sometimes be seen through the viewfinder or on your screen. If your camera has a depth of field preview then using this function closes the aperture to that being used in the shot. When normally looking through the viewfinder, the camera automatically opens the aperture to its widest setting, allowing as much light as possible for you to compose your shot.
The mechanical size of the aperture is controlled by a rotary system involving blades that slide across each other. This can be seen in the images above, particularly the middle image at F8 where you can clearly see the position of the blades. Compact cameras often have a fixed aperture and no settings can be adjusted.