No, it doesn't seem like you are trying to argue. We're both just trying to get at the truth, not to put each other down.


I think you are right about "same light but spread out" point, and that is a good way to think about it. But I don't think the filter size is a good indication of this. For example, the 17-40mm f/4 has at most a 10mm effective aperture when wide open (40mm at f/4 means 40mm/4 = 10mm aperture), so you might guess it has a tiny filter size. But it also takes a 77mm filter. I don't know anything about optics so I don't know why this is true. Maybe it has to do with avoiding vignetting on a lens that has to illuminate a sensor that is large compared to its focal length.