Rikenon 50mm f2
This is the standard Rikenon 50mm. There were many other versions; the 'L' 'S' 'P' 'XR', Sears, etc. Some early models were reportedly made by Tominon. In my experience they are all sharp. The 'P' version is considered to be one of the sharpest 50mm's ever made.
My test lens has a spot of fungus in the center-middle of the lens. As far as I can tell, it doesn't affect image quality. This lens is very sharp.
It handles very well on my M3. Focusing is easy with the viewfinder attachment. Metering is consistent with little deviation from f2 to f8. The Fotasy PK adapter is a bit tight but usable.
The lens has a plastic/metal feel, similar to a Canon 'new' FD lens. Ufortunately, it is not as serviceable as a Canon lens. If you get a fungus problem you may have to live with it.
The IQ of this lens is outstanding. Bokeh is smooth and consistent in even light. Color rendition is very accurate and highlight color is more accurate than most lenses I've tested. Shadow detail is on par with most lenses. Contrast is excellent at all apertures. I haven't tested for flare.
At an average selling price of $25-$35, this lens is an incredible value. Its one weak point is the lack of serviceability.