I did the test with a f1.4 Canon FD EOS 550D. The result is extremely poor: blurry, no sharpness or contrast, photos unusable. I am very disappointed and I do not understand the good reviews from previous buyers.
I have found people who have had unpleasant surprises like me with this kind of ring on this link [...]
I'll return it to the seller but I am curious to know in which case the record is good or bad since we found evidence that go in both directions.