To correct a bit what I wrote before, rather than editing the previous post:


What do you mean, "how does it effect f/stop?" In terms of exposure,
it doesn't affect it, at all. The same amount of light per square
centimeter gets through the lens, regardless of the camera or sensor.


If you're concerned about depth of field, don't use the "crop factor." Instead, go to DOFMaster
and check out his calculator. It takes into account the "circle of
confusion," which depends somewhat upon the magnification when printed,
if I recall correctly.


The reason I said that is that which
camera's f/stop gets multiplied by the FOVCF depends upon what you are
comparing. If you want the same framing with the same lens,
then Mark is right (or, at least, pretty close--the details are a
little bit different). Here's data from DOFmaster for the 5D (full
frame) and 30D (1.6x).


30D, 100mm, 80 ft, f/2.8: 70.8 - 92 ft, total 21.2 ft, 9.2 ft in front, 12 ft behind


5D, 100mm, 50 ft (to get the same framing), f/2.8: 44.3 - 57.4 ft, total 13.1 ft


To get the same DOF total, stop the 5D down to f/4.5:


5D, 100mm, 50 ft, f/4.5: 41.5 - 62.8 ft, total 23.1 ft, 8.5 ft in front, 12.8 ft behind


(Do note the difference in the front and behind numbers--not great, but different.)


That's only one comparison, however. What about comparing the DOF at the same distance
and focal length? In that case, it's the 30D's aperture that would have
to be stopped down--the reverse of the above. Start with the 50 ft,
f/2.8 data above.


30D, 100m, 50 ft, f/2.8: 46.2 ft - 54.4 ft, 8.19 ft total


Stop down the 30D to f/4.5 and you get 44.3 - 57.4 ft, just like the 5D at f/2.8.


Why
is that important? Distance, not focal length, determines perspective.
If you want the same perspective, you'll have to have the cameras at
the same point.


Now, get the same perspective--i.e., distance--and the same framing, by changing the focal length. We could use either camera as the standard, so choose the 5D.


5D, 100mm, 50 ft, f/2.8: 44.3 - 57.4 ft, total 13.1 ft, 5.7 ft in front, 7.4 ft behind


30D, 63mm, 50 ft, f/2.8: 41.5 - 62.9 ft, total 21.5 ft, 8.5 ft in front, 12.9 ft behind


So, to get the same DOF along with the same perspective and framing, the 5D would have to be stopped down to f/4.5, as before.