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Tony,
Why do the ISO200-ISO1600 images look the same on my webpage - even though they are the same exposure? The reason is that I have boosted the luminance on each ISO so that it matches the ISO1600 image. That means the luminance in ISO200 image was multiplied by 8x and so forth. These tests must be done in raw mode.
What I was attempting to show was that there is no more information in the image from ISO200-1600 for a reasonable exposure. On my 300D for exposures of several minutes or more ISO200 seems to record as much detail as ISO1600, although background banding looks a bit ugly and I tend to use ISO400 as a result.
Your 20Da will have different characteristics as it seems ISO800 and 1600 have very low noise relative to 200/400.
Terry
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