Further to my last post about lenses, I did a comparison test today. I started out with my long zoom at 50mm, 300mm, and 300mm with a x2 converter, then tried the mirror lens (500mm f8) and the long lens (500mm f8-f22), all pointed at a block of flats a few hundred yards away.
To cut a long story short, the mirror lens is the worst of the three, with the results blurry and a bit grey. The zoom lens is a little better at 300mm with the x2 converter, better still (but obviously a smaller image) without the converter, but neither is particularly wonderful - not hugely surprising since zoom lenses are always a compromise and tend to give their best results towards the mid range, and converters don't get on well with zooms, but a bit disappointing. I didn't try manually setting aperture etc, which might have given better results. The cheapo 500mm lens did better than either of them at F8, a lot better when I stopped it down to F16, and I think the results would have been better still if I'd used a more solid tripod - the one I have is about 30 years old and getting a bit shaky. The long lens pictures are slightly off level because of another problem; because of the arrangement of furniture in my front room there wasn't room for me to get behind the camera properly with the long lens, so I had to use a right-angled viewfinder and didn't notice it was slightly out of position.
So the verdict, I think, is that the mirror lens goes to eBay and I keep the long lens for occasional shots of birds etc. - probably won't use it much but I don't use the mirror lens much. Its big disadvantage is that MINIMUM focal distance is about 12 metres, whereas the mirror lens could focus to 2 metres, but I don't think I've ever used that capability.
Zoom set at 50mm - the block of flats at the back is the target for the other photos
Zoom at 300mm
Zoom at 300mm with 2x converter
Mirror lens 500mm
Long lens 500mm