I have a Lentar 450mm f8.0 and a 500mm f8.0 mirror lens with some camera shop name on it. Both do a good enough job to get 16 X 20 prints from 35mm. The mirror lens is a little low in contrast, but nothing a no. 4 printing filter won't fix. I say use them and enjoy them. Just hold them still, preferably on a tripod, unless you're panning race cars or something. I've got some pretty good shots panning, and they work fine.
As I remember, the 500mm came out about the same time as the 1000mm. The UK magazine, Amateur Photographer did a test around about 1963 on the 500 and it wasn't at all bad. What struck me was the fact that it was a very simple lens with actually only 3 elements, of which 2 were cemented together and the two groups were at opposite ends of the tube. The 1000 was the same as the 500 but bigger, again the same optical setup.
All, in all it works out the same "F number" (not linear aperture size) across lenses gives the same CoC size on the film.
When the physics of optics gives such an elegant solution, why do some not want to believe it :blink: