I have been wondering on my recent photo outings to the airport, when I should change the ISO speed. I have a 300D btw, with the 75-300IS lens. I seem to be having a slight problem when I shoot with ISO 100, since I set the aperture to 9-11, and I usually still get slightly blurred pictures when I shoot aircraft in action (landing/taking off). This, I believe is due to the fact that the time value gets set too low. Wouldn't an ISO of 200 help? Does the image quality drastically decrease, so that they would be useless for a.net? Any help is appreciated!