Simon I have always been fascinated by this design - dont know how practical it is? might damage/crush???
http://www.astrosurf.com/astroscope/tips/afocal4/ok.htm EDIT: sorry! of course this only for cameras were the lens only moves internally (like the Nikon CoolPix 4500 in the pics)!! I'm not sure about video cameras? is their any external movement of lens? i have no experience with them. The cp 4500 and its earlier versions 995 990 ect. were conveniently designed for also using with microscopes, and were very popular with bird photographers - hence the availabilty of a few quality screw straight on ep's - they screw into the 28mm thread that also exists for adding adapters like the teleconvertors ect. their is a great pic of this set up in Shevill Mathers "Moonlighting" article/column in S&S july/august 05 page 62. he also has a pic of a video camera connector there.
Kearn