Yeah yeah yeah, reviving a dead thread, whatever. This mystery is unsolved. I'm a happy owner of the MD-mount Vivitar 90mm f/2.8, and I'd say john_tribe is correct. As for the lens, it is razor sharp until infinity, where it softens a bit. That seems to be the case regardless of aperture. The lens is definitely at its best below f/8, combining extraordinary detail with a lovely bokeh that is consistent all the way out to the edges of the frame. I have only had it produce a lens flare once out of hundreds of shots taken with it, and even then the flare was weak. I suspect this might have to do with how the front element is recessed pretty far into the housing- kinda like having a built-in lens hood. Comparing it to photos of the Panagor lens: both have 8 aperture blades, and the blades are the same shape. The concentric rings on the front are the same. The markings on the lenses are different, but the placement of the markings is identical. The focusing ring is the same, the proportions are the same, even the QC sticker is identical. If someone owns that Panagor lens, I'd love to compare photos to see if the lenses perform similarly- maybe there's some hidden difference in the optics.