> So it's the diameter of the front of the lens that determines
>the light gathering ability?

Hi-- No, the relative aperture. The aperture may or may not be within the physical lens, and no dimension on the lens may correspond to the aperture
(unlike telescopes, where a simple specification of the mirror or lens
glass diameter is all that is given, in lenses:
focal-length at infinity / max. aperture dimension (which may be virtual) =
relative aperture = max. light gathering ability
i.e., a 200mm lens with a 50mm max. aperture is f4 wide open).
Hope this helps.
David Ruether