Definitions

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

  • n. The ratio of the focal length of a lens or lens system to the effective diameter of its aperture. Also called f-stop.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. 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.

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

  • 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

  • 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)

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

    Further aspiration.

  • 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.

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

    Further aspiration.

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

    Macworld

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

    Macworld

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

    PCWorld

  • The easiest way to control the depth of field is to set your camera to its aperture priority mode and then dial in the biggest aperture (the smallest f-number) that your lens allows.

    PCWorld

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