Relative Aperture of Camera Lens

Relative Aperture of Camera Lens

  • Every lens has a focal length (or, in the case of zoom lenses, a set range of focal lengths). Other than focal length, every lens also has an aperture, which is the opening through which the light -asses to form the image on the camera sensor or film. Camera lenses (other than fixed-aperture mirror lenses) can be set at any of a number of specific apertures.

  • The diameter of the opening is called the relative aperture, and is identified by its f-number (to a degree, the terms are used interchangeably by photographers). Relative aperture is the mathematical ratio between the focal length of a lens and the diameter of the aperture (dividing the focal length by the aperture diameter yields the number). A lens whose aperture is open to a diameter of one-eighth of its focal length is said to be set at f/8; if open to one-quarter of its focal length it is set at f/4; and so on.

  • Along with the shutter-speed setting, the setting of the lens aperture determines the amount of light passing through the lens and thus the amount of exposure given to the camera sensor or film. The size of the aperture is varied by means of an iris diaphragm consisting of several movable metal leaves in an assembly placed within the lens mount, usually between two of the glass lens elements.

  • Lenses are classified for speed (the maximum amount of light that can be passed through them in a given time) and by the size of their maximum opening. Thus, a 50 mm focal-length lens having a maximum opening of 25 mm has a maximum relative aperture of f/2, and is described as an f/2 lens. A lens is said to be fast if its maximum aperture is relatively large - f/2.8 or larger. A slow lens would have a maximum aperture of f/6.3 or f/8 (such lenses are frequently used with view cameras). Lenses falling between these categories are, of course, called medium-speed lenses. Note that, as the actual size of the aperture gets larger, the f-number describing it becomes smaller, and vice versa, a consequence of the proportional mathematics used.

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