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May 27, 2008 - Dave Hecei
A camera lens has two functions. It focuses light onto the sensor, or film, and controls the amount of that light hitting the sensor. The aperture in a lens is an iris and works in a similar way as the pupil in your eye. Your pupil lets in the proper amount of light into your eye allowing you to see in various lighting conditions. When it is very bright out, your pupils get smaller letting in less light. In a darkened room, your pupils get larger to let in more light. The camera’s aperture does the same thing.
Camera lenses have a maximum aperture, the widest opening, and a minimum aperture, the smallest opening. Aperture settings are denoted in f-numbers or f-stops. One of the widest apertures is f/1 while one of the smallest is f/64. A typical modern zoom lens, like the one that came with you’re DSLR, should have an f-stop range of f/3.5 to f/22. Standard ranges of f-stops are: f/1.4, f/2.0, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/64. Each step in these f-number, say f/4 to f/5.6, is considered one stop. Going up one stop lets in half the amount of light, while going down one stop lets in twice the amount of light.
The aperture does control more than just the amount of light coming through a lens. The f-stop also controls something called depth-of-field (DOF). The DOF is the range of sharpness in front of and behind the point of focus. When using a standard lens, one that gives the same magnification as our eyes, this range varies dramatically with the aperture.
At f/2, a very wide aperture, this range may only be about 12 inches. At f/22, a very small aperture, this range may start from 6 feet away from you and go all the way out to infinity. Master landscape photographers, like Ansel Adams or David Muench, use great depth of field to allow them to have everything from a few feet in front of the camera to as far as you can see be in sharp focus.
You can also select a very wide-open aperture and focus precisely on a subject and allow the background to blur out. This allows you to get ride of any distractions that take away the eye from the main subject. It can also give the photo a more dimension by allowing the subject to pop out from the background.
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