I have a 20cm f/4, a Q with John White AI conversion, a QC with factory AI conversion and an AI. My favorite is the QC. The cm has a very long close focusing distance. The Q is fine but the QC is better coated. The AI is supposed to be sharper but I don't see it. It's smaller without being much lighter. The Q and QC seem to balance well on the camera. The AIS is the same as the AI optically but the AIS is slightly different mechanically and might have slightly improved coating. I see very little difference in the effectiveness of the coating between the QC and the AI. There was a 'K' version of the 200/4. It's the same, optically, as the AI but in non-AI form. On a Nikkormat, using a 200/4 is only easy in bright light. With an F2, FE, FE2, N2020 or N90S I use an E screen. There is no focusing aid to black out and the grid lines are a good sharpness reference. Fixed focal length lenses in this range became less popular over time as zooms in the 80-200 range became more popular. In most cases a 200/4 Nikkor from the Q on is sharper across the field than a zoom at the 200mm setting.