A random page of calculated mayhem by fyngyrz.
So, you're thinking about shooting a planet with your DSLR, eh? What's going to happen? Well...
This page allows you to determine how many pixels any planet in the solar system (except earth, of course) will take up in your final image.
The information needed from you is the horizontal resolution of your camera's sensor, the crop factor, the focal length of your lens in millimeters, and the magnification factor of your teleconverter, if you use one. You can also input the size (in arc-seconds) for any astro object other than the other eight planets (What? Yes, Pluto is a planet, those IAU people are nuts.) Once these items are entered, press "Submit."
The primary assumptions made here are that pixels in your camera are square, and that your sensor's aspect ratio is the same as film. Both of these are almost certainly true. If not, I don't know what to tell you other than the results will be wrong.
Here's a tip from Ken Philips: "When photographing tiny objects with a Bayer CFA chip (ALL DSLR's 'cept Sigma), you should try to get at least two pixels per arc second of object size (given that great seeing will allow just under one arc second of resolution) and take many, many shots... using registration and stacking software to combine the good ones."
...and one last caveat; the focal length in mm that you have for your lens may not be quite correct, because some manufacturers sort of fudge those numbers. Consequently, although I use the most precision I can in the appropriate places in the math, the numeric results here may differ by a few percent from the actual results you get.
Lens system total focal length is 400mm