* Annika1980 is quoted & my replies are inline below :

As I understand the "crop factor", it comes from the 20D/30D/40D
sensor being (approx) 1.6x smaller in width and height than a
full-frame sensor.  So when using the same lens at same focal
length, it only captures the middle of the image that a FF sensor
would.

I'm not sure what your comparison is supposed to be showing.

If it's field of view for the same focal length, the first 2
cameras have the same crop factor so should appear the same, and
the third pic should have a wider and higher field of view by 1.6
times.  That's assuming the images on your page are all scaled to
the same image size.

While the 20D and 40D have the same crop factor, the 40D's denser
pixels would allow it to be printed at a larger size for the same
quality print by 3888/3504=1.1 times (rounded).

The 1DsMK3's pixels would allow it to be printed at a larger size
again for the same quality print by 5616/3504=1.6 times that of
the 20D, but because of the crop factor the image will be a wider
field of view by 1.6 times.

So I think what it boils down to is that for prints of the same
quality, same focal length, the 1DsMK3 will give a 1.6 times
bigger print, but if you measured, say, the birds head it would
be the same size in both prints.

For the 40D you'd get a print size 10% bigger than the 20D, and
the bird's head would be 10% bigger also, because it has the same
crop factor.

That's my long-winded take on it all.

The best math geek on the newsgroup is Wild Bill.  Perhaps he can help
you.
Helen

Take two pictures with 20D and 1DsMK3 using the same piece
of serious glass. The center 1DsMK3 cropped to 1.6 of the
original image with give you an image of 3510 by 2340. Dead heat
with the 20D. Looks to me like same pixel density . The only
question is $8K worth it to fill out the frame.

w..

The resolution of the Mk III and the 20D are practically identical at
156 pix/mm.  But of course the Mk III is full frame and getting a larger
scene (for the same FL).

So both images at the same distance and FL and printed (without resizing
or cropping) at a given dpi will yield the same size subject (on paper),
you can just get a larger overall image with the Mk III.

That's just another way of saying you'll need a 1.6x FL increase
everywhere to get the same subject frame at a given distance, but at 60%
higher resolution (Mk III v. 20D) allowing much larger prints.

                Mk III  D40     D20
X pixels        5616    3888    3504
Y pixels        3744    2592    2336
x mm            36      22.2    22.5
y mm            24      14.8    15
300 x           18.72   12.96   11.68   <- Print size, width, inches
300 y           12.48   8.64    7.79    <- Print size, height, inches
Pitch (pix/mm)  156     175.14  155.73

Note that the the 20D and Mk III have 155.73 and 156 pix/mm ...
practically identical.  The 40D has a higher pitch, and we'd expect that
to be a little (not much) noisier.

As to the 40D being only 10 Mpix, that's not much improvement over the
20D's 8 Mpix... about 11% in resolution.  Really not worth it if your
20D is still going strong.

You'll have to go scrounging for a 5D or 1 Ds Mk II to get a noticeable
improvement in overall detail.  Or sell your soul for a Mk III.

The other thing is the Mk III has a 14 bit A/D.  This will improve
(probably) the shadow noise even more allowing very high ISO's.  (I
assume at least that for the usual fiziks reasons it can not go to
higher DR).

And at its "natural" ISO, I'm sure it will give incipient Hasselblad H3
buyers great pause.

Cheers,
Alan.