The F Number or F-Stop is used to express the aperture setting of a photographic lens.
More technically speaking, this number is the ratio (the mathematical division) between the focal length and the physical aperture of the lens. This number therefore has no unit.
It would be too long and tedious to enter into more details here. This Wikipedia page will satisfy your curiosity if you want to know more.
What is important to remember is that the number F is one of the factors on which the photographer can play, within the limits set by the digital camera, to manipulate the exposure of his images and thus modify the results obtained. The lens aperture also influences the final appearance of the image by controlling, among others the depth of field and the time required to capture the image (exposure time).
Large F number = Small aperture
It is important to remember that the greater the F number, the smaller the lens opening is. And conversely, the smaller the F number, the larger the aperture.
Click to enlarge: Standard lens Openings. Image from Wikimedia Commons
F= Denominator of a fraction
A simple way to remember this is to think of the number F as the denominator (the number under the line) in a fraction, with the numerator (the number above the line) always being 1:
Thus for exemple, 1/2 is larger than 1/2.8 (nearly ⅓) or 1/4, 1/8, 1/16, 1/22, etc.
So the apertures scale looks like this:
f:1 ; f:1,4 ; f:2 ; f:2,8 ; f:4 ; f:5,6 ; f:8 ; f;11 ; f;16 ; f:22; f:32 ; f:45 ; f:64 ; f:128 etc
Every time we move to the right on the scale (or increase the F number) of a full stop, we decrease the light received by half.
And every time that we move to the left on the scale (or diminish the F number) a full stop, we double the amount of light received.
On a modern digital camera, the aperture control can be automatic or manual. In manual mode, the setting will be in a menu or via a setting wheel on the AFN housing. Sometimes, on advanced digital camera with interchangeable lenses, this setting can also be changed via a ring surrounding the lens.
Also note that only part of the above scale will be available for a given lens or digital camera. No lens or camera covers the entire range of possible apertures.
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© Charles Martel 2015