Making a Teleconverter

[dnas]

I have a small, front screw-in 2x Raynox teleconverter for one of my digital cameras.

I've been looking at other teleconverters for my other camera (Panasonic FZ10). I've also seen the 5x and 8x converters.

Then I had an idea. I was thinking that I might be able to convert an old 200mm lens from a 35mm SLR into a teleconverter for the front of a digital camera. It would require a concave lens/lens group to diverge the lens, in much the same way that telescopes do. I figured that I could end up with a reasonable quality 3x-5x teleconverter if I select the right concave lense element and distance.

Has anyone tried this or have any comments?