My copy of this lens has a remnant of cleaned and killed fungus on the rear element. I do not think it causes a loss of image quality though. The pics below were taken on a Eos 6D with an M42 to Eos adapter. The lens handles well and it easy to focus on the 6D. This is a special lens. It has a very long focus throw and is strongly dampened. It focuses closer and more precisely than any other standard lens I have ever used. Add an extension tube to this beauty and you'll have an exceptional macro lens. Aperture clicks at half stops thru the entire range. At f1.8 it is very sharp. Vignetiing is noticeable at f1.8 and gone by f3.5. Distortion is noticeable when shooting straight lines. It can be easily corrected. Color rendition and contrast are excellent. Flare is very well controlled because of the Zeiss quality coating on this lens. I was only able to force it to produce one small ghost in several direct shots of the sun. That was at f11. Contrast remains usable across the frame and sharpness is slightly reduced. Bokeh at f1.8 is smooth with a bit of swirl. Bokeh fringing has a slight color shift and the bokeh is a bit busy. In focus fringing is essentially non existent. I simply couldn't get it to fringe. Blown out highlights have very little color shift. Overall, this is a great lens. It's also fun to shoot with. Pick one up if you have the opportunity.