Sunday, August 9, 2009

Glossary; f/stop

The aperture settings of a lens are called f-stops.
The f/number represents the ratio between the focal length of the lens and the effective diameter of a given aperture opening. As the f/number is related to the focal length the f/number is sometimes referred to as the relative aperture.

The f/number equals the focal length of the lens divided by the entrance pupil of the aperture.

Aperture settings are marked in such a way that each position increases or decreases the amount of light passing through the lens by a factor of 2.0 either doubling or decreasing by one half, the amount of light transmitted to the photo sensor.

A high f/number represents a smaller aperture, this stops twice as much light as the previous f/number.
A low f/number represents a lager aperture, this doubles the amount of light from the next higher f/stop.

A camera lens is described by the range of focal length and the f/stop range, IE 18-50mm f/2.8
The f/number in the lens description represents the maximum available aperture opening.

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