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.