Quote Originally Posted by David A. Goldfarb
I've owned a couple of mirror lenses, which I sold when I could afford the FD 600/4.5. The biggest problems they had were low contrast relative to a refracting lens and double-line bokeh in situations that are pretty common in bird photography--water bird wading in reeds, bird in a tree surrounded by branches. A light 500mm lens is attractive for flight shots against a blue sky (bad bokeh not being an issue), but f:8 is on the slow side for that purpose.
I'll add my 2 cents about the Cats. The two that I've used seemed to meter slightly slower than the nominal maximum f stop. Not to mention that shooting wide open allows almost no depth of field. Kodachrome 200 pushed to 400 usually resulted in lower contrast images with these lenses as well as other 400 speed films. Modern emulsions are much better than those I used at the time but you will still be shooting on the razors edge if you want supper fine grained images.

As I said before (actually Dave said it), the Tamron 300 mm maual focus f 2.8 with a high quality 1.4 or 2x tele converter will give you the most bang for your buck if you are working on a tight budget.

Also FWIW, the bar is set so high now (and has been for decades) for nature/wild life imagery that the weekend photographer will be hard pressed to produce or make photographs that rival the best professional work.

But it's still fun trying. Just be mindful of the subject and their environment. I've seen some very obnoxious photohraphers in the field, especially at places like Sanibel Is., FL.