^^ I have always been a bit skeptical of how much a filter actually would effect image quality, but I rarely used filter and when I did while I was shooting film I usually used the "high end ones" just because I could afford it more back then.
I have a Canon 35/2 Concave which I wasn't happy with the performance, it was supposed to be a really excellent lens, but i was getting all sorts of artifacts. After reading this, I took off the cheap, uncoated and old hoya filter that was probably bought in the early 1970's with the lens, still skeptical, I was very surprised how much an effect it actually had. I pulled the glass out because without the filter's ring I can't fit the retaining screw-type hood I have for it, and was also surprised how thick the glass was.
Overall contrast is much, MUCH better now. I still think the infinity-corrected adapter holds the lens back some areas, but it is working much better in others.