Although this thread will be about Canon cameras it belongs here in the "Technical and scientific photography" section because the comparison is largely concerned with
the use of the above cameras for astrophotography.
Long term members may know that I find Christian Buil's web-site
a tremendous resource. The man has done it again with his comparative testing of the 40D, 50D, 5D,
5D Mk II and a high quantum efficiency CCD based on the Kodak KAF-3200ME sensor.
As a taster of what's to come just have a look at these two graphs:
The full article
explains the term "Geometric Quantum Efficiency" and also presents graphs for the "Pixel Quantum Efficiency". It's interesting to see how the 5D shows the age of its design. Accuracy is stated to be of the order of ±15%.
So far so good but to see what the gain is by removing the 40D's IR cut-off filter and to see just how much light is being wasted by using a DSLR sensor instead of
more specialised hardware have a look at the next two graphs:
I hope the above has whetted your appetite. The full article CANON 40D, 50D, 5D, 5D Mark II COMPARISON
contains much more: Recommended.