Thanks I've already looked at the Mir site: the lens diagram relates to the old 5 group 6 element 50/1.8. There is a 50/2 lower down with the same enlarger type lens diagram as the MIJ.
I have just had a OM1n refurbished (by Michael Spencer, highly recommended). I am intending on moving over to Olympus in order to make full use of my Tamron SP 90/2.5 which I find to be an excellent lens. It focuses in the opposite direction to my Pentax lenses and often catches me out, whereas its the same direction as the Olympus lenses.
Prior to committing to the move from Pentax to Olympus, I thought I would compare the Oly lenses to the Pentax lenses. Both the OM 28/2.8 and OM 135/3.5 I have seem to perform very well, even in comparison to my Pentax M 28/3.5 which is a very sharp lens, and also in comparison to the Tamron SP 90/2.5 which can act as a control, as it can be mounted to both the Pentax and Olympus cameras. Looking at my test negatives (x25 scope under a Nikkor EL 50/2.8N in the enlarger) I was struck by the MIJ being softer than the Pentax 50/1.7. In order to exclude the possibility of mucking up the test, I will simply shoot the test again.
The significance of this is that I don't think I can print larger than 5x7 with the MIJ shots and have a sharp photo, whereas 12x16 would not be out of the question with the Pentax M 50/1.7.