Mirror Lens?

I have one of these. It's not bad for such a cheap lens - it gets a bit soft at the long end, but keep it down to around 200mm and stop it down a bit and it performs pretty well. Vertainly if you're on a budget and want a telephoto, I think it's a good choice.

Here I'm back again.
I guess I was too busy promoting the mirror lens instead of giving an answer.
To use the lens you need to do 3 things :
1) Use an adaptor ring to go from FD to EOS mounting
2) Place de selector knob to Av
3) Place the light correction setting (Av+/-) all the way to -2
That should do the trick. It works for me anyway.
Good luck with your choise.
Alain

Yes,as some others have said,there are good types of mirror lenses.While they still have the typical mirror drawbacks -odd bokeh,no aperture adjustment,etc ,they do have the benifit of insanely small size

Where do these fit (optically) compared to other lenses?

At the bottom are the cheap mirror lenses with low sharpness and contrast

Next would be the cheaper x-300 telezooms (used at the softer long end)

Then the very inexpensive 'preset' manual focus tele lenses that i linked before.Also about equal to these are the better x-300 zooms

About here is where the better brand mirror lenses fit.
Also here i presume would be the good brand superzooms such as the sigma 50-500 (but i am guessing at that i have no experience with these zooms)

I suspect the adapter you're using is Canon's FD-EF adapter, which applies a 1.25x TC effect as well? Getting one of those and a Canon 500 mirror will cost the interested party close to enough to get a Bigma instead, and get about a stop faster lens that has AF and full aperture controls. If there are any other optical adapters that don't significantly degrade IQ, we'd love to know about them.

I've got a 500mm mirror lens... It works, but it's really not the best solution. For one thing, all your OOF highlights will be bright donuts of light... Talk about horrible bokeh. Another drawback is that they have only one aperture, usually f/8 for 500mm lenses. This gives you a very dark viewfinder. They also don't have AF - you have to manually focus with aforementioned dark viewfinder. Yuck.

One thing that IS nice about these lenses are that they're ridiculously small and lightweight for their focal length. So if super portability is your main criteria, they'll fit the bill. Otherwise...

Keep looking!

I just posted my review of Rubinar 1000mm mirror lens on Canon 5D body if you are still interested.
https://photography-on-the.net …php?p=3604032#p​ost3604032