Based on Bill Claff's data, The canon 30D has quite low readind noise. Any camera after that the read noise has been at least doubled up, includind the 1D-X. Why??? Is Canon is going backward on the read noise???
It is because you are comparing 12 bit to 14 bit files.
At Building Panoramics we shoot a lot of skys for our library and of all the Canon cameras we've used the 30D was by far the worst for noise, in fact worse than the 20D for some unknown reason.
With regard to
Actually it was Privatebydesign's link. I can't find the practical link with Claff's data. So the 5D mk2 has the same read noise at ISO 640 as it does at 100 ? Yet shoot a sky at ISO 640a and it's full of noise whereas 100 is ( reasonably) clean. Likewise ISO 125 is similar to 100.
If you're exposing in total darkness with the lens cap on surely you are under exposing never mind how long you leave the shutter open. What about heat generated at these long 'exposures-that-aren't' ?