|
The "f-stop" or focal ratio is simply the ratio between focal length and aperture of a lens or mirror.
Unlike most telescopes, most camera lenses are designed to be stopped down in steps. The series obtained by multiplying the f-stop number with sqrt(2) gives a sequence of exposure values in steps on 1 EV (all else being equal). This means that stopping down from f/4 to f/5.6 has the same effect as halving the exposure time or the numeric ISO/ASA sensitivity of the film or sensor.
With camera lenses the f-stop sequence starting at f/1 is pretty much hard-wired into the minds of photographers, but it is no more than a convention. With telescopes there is no need for any such convention, so the focal ratio of a telescope denotes nothing more than the ratio of its focal length and aperture.
Cheers
Steffen.
|