A month or so ago I was looking at either the Canon 200mm f2.8 or 135mm f2.0 lens, but changing priorities has put that on the back burner for a while. So now I am looking at a couple of options which are much cheaper to at least get my toes wet until I can afford the 135mm f2.0.
My question is regarding light through put as it affects exposure time. Is the f-stop the only determination? That is, if I got the 35mm f2.0, would I get a good feel for how long I need to expose?
I have some difficulty with this as the 135mm lens has an obviously larger aperture. A thought experiment with two different funnels in the rain draining into the same sized cup being analagous to a light cone hitting a CCD would suggest that the larger aperture would result in greater capture and so shorter exposures. Of course, taking that idea a little further, a steeper funnel would also result in a shorter path for each individual drop at the edges of the funnel, again resulting in the cup filling up sooner.
So is exposure length tied to both aperture and f-ratio? If that is the case, then we would end up in the curious situation where exposure length would then be a function of aperture diameter^4, an odd function indeed.
