To use Contax/Yashica mount lenses on a Canon EOS camera body, the adapter has to incorporate a thin negative element to correct for the extra extension so as to restore infinity focus. This works as a weak tele-converter and will indeed have a certain impact on the absolute image quality, If it is possible to use a plain adapter to fit Brand X lenses to a Brand Y camera without the need for such optical makeshift, I have no problem with that, otherwise I would not be at all keen to do so.