Proxars introduce ALL (!) possible lens abberations (the ones your revered Zeiss desgiers painstakingly designed away), reducing your expensive lenses to the level of some less expensive ones.
Alll? ALL(!) ... ???
Plain beef by-product. They may increase the amount of certain abberations (or may not...) but INTRODUCE??? ALL possible? Certainly you would not include Coma (a possible abberation), would you?

So if you think it is a good idea to use these things still, you might also want to remind yourself why you have spent the amount you have on getting good lenses to begin with, and ask yourself which of the two decisions - use Proxars or spend enough to get good lenses -was the silly one.
I've never regretted the purchase of my Hasselbad gear - lenses OR Proxars.

However, the discussion was about a comparison of Proxars and extension tubes/bellows ...
You do not intend to change the subject ... do you?

You better brace yourself - I do NOT intend to abandon my use of Proxar supplementary lenses - not any time soon. They work *fine*, as far as I am concerned.

. Bellows...