Why do point light sources behave differently from diffuse ones?

Hi

I've just discovered The Naked Scientist and Dr Karl shows. Superb stuff.

I have a question...

In photography, by adjusting the f-stop you can alter the exposure for a given shutter-speed. So for example, a 500mm f2 lens shot at f2 will give the same exposure as a 100mm f2 lens shot at f2.
But in astronomy, where stars are point light sources, the brightness is related only to the size of the aperture, so the 500mm f2 lens which has a 250mm aperture, will create a brighter image than the 100mm f2 lens which has a 50mm aperture.
Why is it that point light soures behave differently to diffuse ones? And is this always true or are there any exceptions? (eg. infinity focus vs. micro-scopic focus / super-macro?)

Kind Regards
Gordon Price