f-number

IPA: fnˈʌmbɝ

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

f number

IPA: ˈɛfnˈʌmbɝ

To do this, choose a small aperture, which will be denoted by a larger f-number.
On a camera, the f-number is usually adjusted in discrete steps, known as f-stops.
In addition to considering focal length, pay attention to the "speed," or f-number, of your lens.
First, it's tempting to just dial in the largest f-number you can, but this is usually a bad idea.
Each "stop" is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop.
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).
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!!!
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.
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.