Definitions

from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun The ratio of the focal length of a lens or lens system to the effective diameter of its aperture.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun photography The ratio of the focal length of a lens to its diameter; a measure of the light-gathering ability of the lens.

Etymologies

from The American Heritage® Dictionary of the English Language, 4th Edition

[f(ocal length) + number.]

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Examples

  • Blaker explained this in another book, Field Photography (1976): ... at significant magnifications the choice of f-number is ... nearly always a matter for compromise between depth of field needs and the resolution of fine detail in the image.

    Archive 2009-06-01 AYDIN 2009

  • If your camera has manual controls, you can add more light to your shots by opening the aperture more widely (using a smaller f-number) and slowing the shutter speed (to, say, 1/125 or even 1/60 of a second).

    Tips: 5 digital camera mistakes to avoid 2010

  • If your camera has manual controls, you can add more light to your shots by opening the aperture more widely (using a smaller f-number) and slowing the shutter speed (to, say, 1/125 or even 1/60 of a second).

    Tips: 5 digital camera mistakes to avoid 2010

  • Blaker explained this in another book, Field Photography 1976:...at significant magnifications the choice of f-number is...nearly always a matter for compromise between depth of field needs and the resolution of fine detail in the image.

    Depth of field, f-numbers and diffraction (or how to take a rotten photograph) AYDIN 2009

  • Each "stop" is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop.

    Further aspiration. 2006

  • The literal interpretation of the f/N notation for f-number N is as an arithmetic expression for the effective aperture diameter (input pupil diameter), the focal length divided by the f-number: D = f/N. Noooo!!!

    Further aspiration. 2006

  • On a camera, the f-number is usually adjusted in discrete steps, known as f-stops.

    Further aspiration. 2006

  • To do this, choose a small aperture, which will be denoted by a larger f-number.

    Macworld Ben Long 2010

  • First, it's tempting to just dial in the largest f-number you can, but this is usually a bad idea.

    Macworld Ben Long 2010

  • In addition to considering focal length, pay attention to the "speed," or f-number, of your lens.

    PCWorld 2010

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