What is an aperture on a camera?

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The aperture is an opening within the lens whose size is controlled by a series of overlapping blades. It's sole purpose is to control the amount of light that reaches the film or digital sensor during the exposure time. Anything else, such as depth of field, is a spin off of its function.

The size of this opening is controlled by settings on the camera shutter speed/aperture dial or on the lens. Many cameras do not have the ability to "stop down" the aperture before the moment of exposure, so it is not something most people can observe. At the moment of exposure, the blades move to create an opening within the lens just before the shutter opens to control the duration that this occurs. Together, they control the exposure.

We quantify the size of this opening with F numbers. Simply put, the F number expresses how many times an aperture will fit into the focal length. Thus, in a 100mm lens, F4 means it will fit in 4x and will be 25mm in diameter. In a 200mm lens, F4 represents an opening of 50mm. This explains why longer length lenses typically have larger maximum apertures, and why the higher F number represents a smaller aperture (F16 must fit into the focal length 16x, thus it has to be smaller).

The answer moved to the discussion page, while good, was more about the effect of the aperture and really belongs in a answer to a question about depth of field.
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