[AussieBill (Bill)]

Given that

F = Focal Length / Aperture

Of what use is this number and why is it important to me ?

I would prefer to know the Aperture and the focal length of an instrument and work out the F number if I ever need to know it.

What do we use the F number for ? i.e.

High F number telescopes are good for ..............

Low F number telescopes are good for ..............

Did anybody ever wake up in the morning and suddenly say "Aha - I need an F4.6 that will do the the job ! Its obvious, why didnt I think of that yesterday " ?

Bill