Question on Scopes (and other observation devices)

[pmulcahy11b]

Anyone know what the maximum observation range on a scope with a given power and focal length would be? I only need one -- I can come up with an algorithm to figure out the rest. (I doesn't necessarily have to be the same range as the weapon; I would guess one could see further than he can hit with a scope.

I'm working on some scopes right now, that's the source of this question.