"Eric Miller" wrote in message
...
I went from the 10D to the 5D. When I had my 10D, I learned to like the
1.6x way of fooling myself into thinking my 400mm lens was a 560mm lens.
Now I am thinking of getting myself another birding camera
and am trying to figure out if there is a way to think of resolution as
effective focal length versus the 10D. 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.

You could play square root games, but the easiest thing to do is to compare
the vertical (short) direction pixel counts.

The 7D is 3456 pixels in the short direction.
The 10D is 2048 pixels in the short direction.

So the 7D (cropped to 6MP) is like putting a (3456/2046)x (that is, a 1.6x)
TC on your 10D.

The only problem, though, is that nothing's for free. Any infelicities in
your 400mm lens will be magnified by the extremely fine pixel pitch on the
7D. To get pixel sharp images from the 7D, you need to project images onto
the sensor that are "1.6 times better" than the images you are now putting
on your 10D's sensor. That may mean a bigger tripod and not shooting wide
open.

By the way, I think you'll enjoy the 7D. The 5D2 was a noticeable
improvement over the 5D, and the 7D has a few niceties beyond the 5D2; it
should be a great camera.

--
David J. Littleboy
Tokyo, Japan