Misha - you posed an interesting question. I have not done any real world testing with my Canon 20D, but I will give you what I think based on shooting at different ISO and long exposures. With the Canon 20D there is almost no noise difference in ISO 100 and 200. So with your example I would say a) if you could tell any difference at all? I think both the ISO or time difference would have to be much broader to see difference. A shot with ISO 200 shot at 1/15th of a second and one shot at ISO 100 and 8 secs might start showing more of a difference. If you could see the shot of the church at original size you would notice very little noise and it was shot at 25 seconds and ISO 200. Of course digital camera's vary in their ability to shot night exposures. The Canon 20D is an excellent camera for low noise.