Originally posted by hmirchev for some reason it doesnt let me attach images thru flickr, but if you right click the image icon and go to view full image or something similar, it should pop up.
Well, it does not look like diffraction issue for me. I have just one question to rule it out. Is it equally blurry at infinity at other f values?
Now about other possibilities.
Are you using a flanged adapter like
this one?
Does the screws on the lens seem as it has been opened before, maybe the focusing helicoid was not assembled the right way?