What is the relationship between f-stop and ISO? Two lenses like the Oly 17mm 2.8 and 1.8. What ISO on the 2.8 version would get me approximate "speed" of the 1.8? At ISO 200 on the 1.8, would the resulting ISO on the 2.8 need to be 400, 800? With sensors handling low light so well now, is it feasible to have a "poor man's" fast lens in the Oly 2.8 and still be able to get the necessary speed? I know that other factors come into play here. 1.8 DOF > 2.8 DOF. But what else do I get in the 1.8 related to IQ?