Some very interesting points, thanks .
Originally Posted by Daniel Browning
I must have started writing my reply when you posted your original reply, I only just noticed it, sorry. I don't understand though what you mean by the 8MP on hte 30D being the same as the 21MP on the 5D2? 8MP x 1.6 gives 12.8MP, so the 5DII has another 1.6 times the pixel density as the 30D by my rough calculations. Surely that means you can make a 1.6x crop from the 5D2 twice over and still get the same resolution as the 30D? (one of the crops taking you from FF to APS-C equivalent)
And I see your point about ISO100 vs ISO400, but what about ISO800 vs ISO3200 for example? Surely at that threshold the read noise increase will be greater?