[Shiraz (Ray)]

Hi Marc. I'd be a bit wary of the rain analogy - like all simplifications, it can be a little bit misleading.

As Erwin suggested, light definitely comes in from a range of angles. The light that has a wavefront parallel to the aperture is focused by the optics into an on-axis point. Anything that is focused off-axis comes in with a tilted wavefront - it comes at a different angle from a different part of the sky and is focused to a point away from the optical axis in the focal plane.

An image with a million pixels samples the light coming from a million different directions - that's how an image is formed. The pixel size and focal length determines how big an area of sky is looked at in each of those directions - for a given aperture and pixel size, the faster the scope, the bigger the solid angle of sky looked at by each pixel. So the faster scope will collect more light in each pixel (for a given aperture and pixel size). Thus, the amount of light getting to each pixel is determined by both the aperture size (how many photons can get through it from any direction) and also the area of sky sampled by each pixel (how many photons you start out with from any direction). The rain analogy completely misses the second bit and wrongly leads to the conclusion that aperture is all that matters - aperture is fundamentally important, but it is definitely not the whole story..