Cameras designate aperture settings by what is known as an f-stop. This number is written with a lowercase "f" followed by a forward-slash, such as f/22. Aperture controls the amount of light that creates an exposure on the film or digital sensor through the use of a diaphragm whose diameter you can control. The term "stop" without the "f" can refer to either a change in the aperture or shutter speed by one of its designated amounts.
Depending on your type of lens and whether you are using a film or digital camera, f-stop numbers vary. Standard f-stops for a film camera include f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and f/32. Some cameras have half-stops or even third-stops. These half or third-stop numbers may be displayed digitally on a camera with an LCD panel, or they may just be available on an aperture ring as a click between where two standard f-stop numbers are printed.
The lower the f-stop number, the wider the aperture's diaphragm is and thus the more light that reaches the film or sensor. Each f-stop is designed to let in either half or double the amount of light of the f-stop before or after it. For example, f/8 lets in double the light of f/11, but f/8 only lets in half the light of f/5.6. When a photographer refers to "stopping down" his aperture, he is referring to reducing light by changing f/11 to f/16, for example
Lower aperture numbers let in more light through a wider diaphragm, creating what is known as a shallow depth of field. This means while you may focus on a certain plane in the image, the other planes will be less in focus. This is because the wide diaphragm allows light to pass through many of the varying thicknesses of the camera's lens, pushing some planes out of focus. A high aperture number creates a tiny opening in the diaphragm, concentrating the rays of light through fewer varying-thickness layers in the lens and bringing more depths of the image into focus.
Relationship to Shutter Speed
Aperture f-stop numbers work in proportion to shutter speed stops. Each stop of exposure with shutter speed also lets in double or half the light of the previous setting. For example, your camera's light meter may indicate that f/11 and 1/250 of a second create the perfect amount of camera exposure for a certain lighting situation. Changing the shutter speed to 1/500 lets in a full stop less light than what is needed. You can compensate for this by changing the f-stop to f/8 which lets in a full stop more light. For this reason, it is helpful to learn standard f-stops and shutter speed numbers so you can calculate how to create the same amount of exposure with different settings.
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