Originally posted by mithrandir Given similar construction and equivalent glass, I would expect a larger diameter lens to have more light gathering capability. At least this was true of astronomical refractor telescopes.
If construction was similar, that would mean, the focal length to be similar. In that case a bigger diameter would also mean larger aperture and more light gathering power.
Astronomical telescopes must be viewed completely different from photographic lenses, because (apart from the basic geometric optics) they have different applications.
Light gathering power in an astronomical telescope is sure defined by its sheer diameter. But what does that mean? It would mean, that a scope with 30 cm in diameter gathers more light, than one with 20cm diameter.
BUT now the focal length comes into the play. If the 30cm scope has 300cm focal length (a 1:10 refractor for example or the typical Schmidt-Cassegrain), but the 20cm scope only has 80cm focal length (1:4 focal ratio, typical for the current fast Newtonians), the astronomer will view these scopes differently:
The bigger scope has a slower (much slower) f-number. So, despite having obviously the larger diameter, it will show much less of a faint astronomical nebulae, which the smaller 1:4 instrument makes easily visible.
The situation is different for point sources (= stars), where the pure diameter, independent of the f-number plays its strength. Here the 30cm scope will show much fainter stars (and accordingly much more stars), than the 20cm scope, because the focal length needs not to be taken into account with these point sources.
In photographic lenses, the light gathering power is ALWAYS determined by the focal ratio or f-number or max. aperture, because we simply do not image point sources, but always objects spread about a certain area. Thus a 300/2.8 with is app. 10cm front lens will exactly show as much, as a 50/2.8 standard lens, with ist tiny app. 2cm diameter.
There is one other and - for astronomers above a certain beginners stage - much more important thing connected with diameter - and that is resolution. The resolving power of a telescope ort lens is directly determined by its diameter. The bigger the lens' diameter, the smaller the objects I can resolve. This is true for the telescope, where a 40cm instrument will show you much more detail on the moon's surface, than a 10cm beginners scope and this is also true for photographic lenses. But the latter will usually be limited in their resolving power by aatmospheric distrubances etc. anyway.
Ben