You could also use a 1.4 or 2x Mutar. The near focus distance would stay the same as it is with the original lens. This would give either the equivalent of 112mm or 160mm. If stopped down one stop the quality should be quite good which would be the equivalent of f8 to 11. As far as the 150mm C lens I would not be so much worried about the quality of the single coated glass as I would be about the Compur shutter. The Compur was a very nice shutter. It is also subseptible to hanging up, if not much used and particularly if used in cold weather and could easily cost a $100.00 to service. I guess if I wanted a lens for portraiture my vote would be for the 180mm due to its performance being better wide open than the 150mm..which is no slouch. My only concern with the 120 S planar would be if I had to work so close to the subject as to forshorten the features. The 160mm CB lens is also nice. Frankly, none of these would be other than a fine choice.