The diameter of a lens’ aperture at a given setting is its f-stop. Each whole f-stop number represents a doubling or halving of the amount of light that is allowed to reach the camera sensor. Lower f-stop numbers are associated with larger lens openings.

The full stop f-number values are:

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/90, f/128

For many zoom lens the f-stop changes as the lens zooms. For example the Nikkor 28-105mm 1:3.5-4.5D lens means that the highest aperture (lowest number) at 28mm is 3.5, but when you zoom the lens to the 105mm the highest aperture is 4.5, meaning that even though you did not change the aperture the physics of the optics does this automatically, therefore as you zoom in on the subject you will lose shutter-speed to compensate.

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