Eric Miller wrote:
So, for example, if I were to get a 7D at
18 megapixels how would that compare to 10D resolution wise in terms of
what focal length lens would I have had to put on the 10D to get a 5
inch tall bird at 20 meters (or any distance) to be rendered by the same
number of pixels (one dimension only or my head will hurt too much) on
the 10D that it would be rendered on the 7D using the 400mm lens.

If we assume that sensors in both cameras have the same physical size
(and they basically do), then all we need to know is the size of each
sensor in physical pixels. I don't know the numbers for 7D, but we can
just assume a square pixel (for both sensors) and use the square root of
the total physical pixel count instead

10D has a 6.5Mpix sensor. Sqrt(6.5) = 2.55
7D has a 19Mpix sensor, Sqrt(19) = 4.36

The above immediately means that given the same bird and fixed distance
to the bird (as you requested in your message), you'd have to increase
the focal length of 10D lens by a factor of 4.36/2.55 = 1.7. If you are
using a 400mm lens on 7D, the 10D would require a 680mm lens.