The above title is not very accurate but I'm not sure how to improve it.


I'm getting the 5d II and am struggling between the 24-70/2.8 vs 24-105/4 IS. I've owned the 24-70 but needed IS, I like my 17-55/2.8and hesitate to go to f/4. I belive that the reduced noise of the 5d II (vs my 40 and 50d) may compensate for that?


In other words, is there a relative rule of thumb when comparing ISO to aperture settings? I see in the reviews on this site something to the effect that going from 2.8-4 lets in twice as much light? At what ISO change would this be equal so to speak? Low light sports are what I'm thinking, with or without a flash.


I realize these are somewhat stupid questions, but I really need IS (wobbly) and low light capability. If simply going from say, ISO 100 to 200 makes for the same relative exposure, I should be good with the f/4. The $390 price difference is significant too.


I suspect the obvious answer is to mount my 70-200/2.8 ISon the 5d and keep the 50d/17-55 EF-S. That's an appealling choice but I wouldn't mind lugging only one body around rather than two.


I'd like to thank all who take the time to read this uneducated and somewhat rambling post. Thedigitalpicture.com has been very helpful to me through the last few years.


gary