From what research I've done, that I-61 LD can be very good. I found a comparison between the J-12 35/2.8 and a fairly recent Leitz 35, and while not identical, that Soviet Biogon stood up pretty d@mn well! I know, I've caught what might be an unhealthy fascination with Soviet lenses; I bought a J-8 for a Canon IIb that I will be restoring. I've yet to see negatives from it, but if it performs like the old Zeiss Sonnar I had I will be very happy. As for the build quality of the J-8, I'm impressed. The focussing is very smooth (without regreasing the helicoid), and there is no play in the helicoid. The fit of the lens' threads to the mount on the camera is excellent and the index lines up correctly. I had to remove enough old separated grease from the aperture mechanism to grease several of these, aperture and helicoid. I also degreased the aperture blades and blackened them with a magical marker. The internal glass surfaces had a very very stubborn lubricant haze, almost like a plasticiser haze, which I finally shifted with trichloroethane. I want to add clickstops to the aperture ring. I'll also improve the internal blacking of the lens mount. I'll sort out the focussing issue next, but I have a feeling that for the way I will be using it, it may not be an issue. This lens was made in 1975, and after inspecting the screwheads with a loupe I'm pretty sure I'm the first one to mess with it.
Originally Posted by Ken.Cartouche