Aperture settings are called f-stops and indicate the size of the aperture opening inside the lens. Each f-stop lets in half as much light as the next larger opening and twice as much light as the next smaller opening. From the largest possible opening to increasingly smaller ones, the f-stops have traditionally been f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45. No lens has the full range of settings; for example, the standard lens on a digital camera will range from about f/2 to about f/16. Notice that as the f-stop number gets larger (f/8 to f/11, for example), the aperture size gets smaller. This may be easier to remember if you think of the f-number as a fraction: 1/11 is less than 1/8, just as the size of the f/11 lens opening is smaller that the size of the f/8 opening.
How wide you can open the aperture, referred to as its "speed," depends on the len's maximum aperture (its widest opening). The term "fast lens" usually applies to lenses that can be opened to a wide maximum aperture for the focal length. For example, a lens with a maximum aperture of f/2.6 opens wider, and is faster, than a lens with a maximum aperture of f/4. Faster lenses are better when photographing in dim light or photographing fast moving subjects. With zoom lenses the maximum aperture changes as you zoom the lens. It will be larger when zoomed out to a wide angle, and smaller when zoomed in to enlarge a subject.
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