[Finite (John)]

Hi all, my poor old magnitude 25 brain is struggling with something I read in 'The Backyard Astronomers Guide'. Disussing Focal Ratio it says fast focal ratio scopes are useful for imaging, but when using the scope visually image brightness is only due to aperture and that focal ratio has nothing to do with it. Can anyone explain to my why that is? (please!)

cheers, John