Does this lens fir onto a 20d cannon?
Praotor 500mm lens Sony Olympus Canon Nikon PK M42 fit
Not without a M42 to Canon EF adapter.
Even then you will be shooting manually all the way
Yes it does through an M42 adapter. Can I suggest that.well it depends. It means going full manual. M42 comes out of a time that camera's we're pretty much full manual. No fancy contact points that allow for auto focusing, the lens reporting the the camera or anything. It is really manual.
So you found a cheap 500mm lens. Yeah M42 screw mount lenses are cheap for a reason. The glass is good and if you take the time to learn working with them you can get wonderful results. Yes you don't really have anything left automatic but light metering. Yeah. It really is a slow kind of photography!
Unless you are used to slow photography I don't suggest it. M42 as great as it is. Is simply not for everybody.