[ving (David)]

... and one that i should by rights know the answer to...

200mm lens with a front lens size of 52mm @ f/9

600mm lens with a front lens size of 72mm @ f/9


are they going to offer the same brightness at the same shutter speed or will the greater sixed front lens offer more light gathering ability?

I really should know the answer!

i know how it works for telescopes, but how does it work for camera lenses?