Bought as a mechanical protection of the optics of my Canon 50mm 1.4 USM filter. Massive problems with the sharpness, visible especially at wide-open aperture (at closed aperture through greater depth compensated): The "targeted" object was blurred, earlier were all the details so crisp. I was about to return the lens or to lodge a complaint as a case of warranty, after tests with 3 different cameras (EOS 350D, EOS 20D and EOS 60D) were similarly bad, especially when AF, and I to an optical error (Linsenjustage) at Lens thought. When I then still down unscrewed the filter, the photos were suddenly perfect. Counter check with a HAMA UV filter on this lens yielded no visible image degradation or shift the focal plane. Am extremely disappointed, but unfortunately the filter no longer return because I have waited too long.
