Just for the hack of it, I have included this lens which is built for the Penacon 6 type cameras for the test. I found the color and resolution to be the most superior of all the lenses I have tested on that day. At 2.8, it is sharper than its M42 Siblings. I managed to get a K-6 to M42 mount adapter quite cheap locally in HK to get this to work.

On a side note, this lens is the best 80mm 6x6 lens that I have ever seen, I have used this on a Pentacon 6 and Mamiya 645. It even out performs my Rollei Planar 80 2.8 for my Rollei 6008 and the Planar 80 2.8 for my Hasselblad. As for the price, it is dirt cheap, it only cost me HK$700 (about US$85) for a brand new one. This lens is quite accessible on e-bay, sometime you can get one with a Penatcon 6 TL at a bargain price.