George Lepp once gave a lucid explanation for the use of teleconverters. His explanation was that with a 2X teleconverter, under ideal conditions, you might suffer a 20% loss in quality. Without the teleconverter you would lose 50% because you would have to enlarge more. Decent teleconverters were made in M42 mount but you have to be patient in finding them. Komura made a 7 element 2X. For macro use the Panagor Auto Macro Converter, with fewer than 7 elements, is also very good. I think there was a Soligor 7 element 2X as well. M42 mount lenses are so low in price now that it might only make sense to use a 2X with a very long lens.