camera basics - What does the f-stop printed on the lens refer to?

I have read this,so I know what an f-stop is, but what does the number on printed the lens signify?

Is this is the maximum f-stop, the ideal, or something else?

Answer

It refers to the maximum f-stop (which is defined as the ratio of focal length to effective aperture diameter). Smaller number means larger opening and more light:

Source: Wikipedia

Some (possibly overgeneralized) examples:

• f0.9 to f1.2 is exceptional


• f1.4 is very fast, usually the pro primes with less than 100mm focal length have this aperture


• f1.8 to f2.4 - still quite fast


• f2.8 - good pro zooms and long pro teles usually have f2.8 constant maximum aperture (constant only applies to zooms, see below)


• f4 - good consumer-prosumer zooms and very long pro teles usually have f4 constant maximum aperture


• f4-5.6 - aperture range denotes variable aperture zooms, meaning you lose light when zooming in (70-300 f4-5.6 is f4 lens at 70mm and f5.6 lens at 300mm)

Read more about aperture from Wikipedia.