1) Point of focus. I focused all lenses manually, so there could be some variation in the exact point of focus.

2) Contrast. Probably due to the Pentacon 6->Canon EOS adapter, and particularly at smaller apertures the Pentacon lenses exhibit lower contrast than the Canon zoom lens. When I conducted this test previously, most of the Pentacon lens shots were totally washed out. Since then I affixed a baffles inside of the adapter and also flocked it to reduce internal reflections. The difference between shots taken before and after my work on the adapter is quite large, but probably wasn't enough to completely kill all internal reflections.

3) Representative sample. I am using my own equipment for these tests, some purchased new but other pieces purchased used. I do not know the history of the lenses I purchased used, and so some of them may have been disassembled or serviced prior to their coming into my possession. At any rate, the lenses tested here are assumed to be a "representative sample" of what is commonly available. Variations in manufacture among other things may skew the results, and so these test images may or may not accurately reflect what you can expect from a different lens specimen.