Playing around with my new (to me) SMC Pentax 1:8 f=1000mm lens tonight. Took some shots of the nearly full moon with the 1000mm, then took more shots with the Bigma (500mm), Bigma plus 1.4x teleconverter (700mm), and then for fun, I put my Olympus E-P1 on the 1000mm lens.
All shots are uncropped so you can see the relative magnification/appearance. ISO was set at 200, and the aperture for the 1000mm was f/11. Aperture on the Bigma was f/8 (didn't adjust is with the TC, so effective aperture changed). If you want to see the full sized shots, visit my Flickr stream. Shots were taken over a ~30 minute period right after sundown, so that's why the light is different in the shots.
First, the 1000mm on the K20D:
The K20D with the Bigma:
The K20D with the Bigma and 1.4x teleconverter:
And finally, the 1000mm on the Olympus (it looks like a rear lens cap on this thing):
Here's the setup. The Pen will actually fit inside the lens cap of the 1000mm.