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Glossary: F-number

The F-number assigned to the camera lens indicates the brightness of the lens or the maximum amount of light the lens can direct to the camera’s image sensor. Both the maximum aperture size and the focal length of the lens play an important part in determining the F-number.

Iris and Focal Length

The maximum aperture size is self explanatory. The larger the iris opening (aperture) at its widest point, the more light will pass through to the sensor. When considering focal length, a lens will direct more light to the sensor with a short focal length (wide angle) as there is more of the physical scene captured by the lens to generate more light. Using a long focal length provides a magnified view of the physical scene and because the amount of the scene which is physically captured is reduced then less light from the scene is captured. Technically the F-number is worked out by the focal length divided by the maximum aperture or:

F-number = f (focal length)/D (maximum iris opening)

Simply put, the smaller the F-number of a lens, the greater amount of light is passed through to the sensor meaning that the camera will provide better picture quality in low-light conditions.

Published on September 18th, 2008 by Greg

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