should we start a 24-70 f/2.8 L *IS* petition?

[Tom Alicoate]

Jon,


I think the 17-55 2.8 is able toget the same amount of light onto the cropped sensoras the 24-105 is on the larger one. They both use a 77mm filter so I am guessing they allow about the same in. The 17-55 just directs it into a smaller image circle. The light on that smaller circle will be brighter per unit area than the 24-105, hence 2.8.I think the ISO vs actual noise argument holds some water, I need to think about that one a bit. I am really not trying to argue and apologize if it seems that way.


Tom

[Jon Ruyle]

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.

[Colin]

I think the 17-55 2.8 is able toget the same amount of light onto the cropped sensoras the 24-105 is on the larger one.
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Not exactly, but close. a 1.6 FOVCF is like a 1.6x teleconverter, in terms of less light. 2.8 would be the same light on a 1.4 FOVCF sensor as a f/4 on a full frame, but 1.6 means smaller, so slightly less. In terms of what this really means, I'd defer to actual results.

[Jon Ruyle]

I doubt anyone can tell


And if they can, differences in the results are likely due to other differences between the cameras and lenses. Of course, we expect better iq on the larger format, but thats a separate issue.