I am just about to buy a MPP VIII which comes with the above lens.
I understand that by removing the front element of the lens it can be used as a 265mm lens but is quite soft and requires stopping down. I am interested in using it at 265 for some portraits. I will most likely be taking these using available light (with a reflector or two) using HP5. Obviously I don't mind a little softness in the image but any advise on what f stop is the minimum before it becomes noticeable would be useful. I would rather push the film than get the image too soft to make a good print.
I believe this has been covered here before - repeatedly.
Wide open (f:11) the converted lens is quite visibly soft in the corners of a 4x5" film, at f:45 it's about as sharp as any other lens. The sharpness improvement seems to me to be quite linear - and I wouldn't hesitate to use it for landscapes at f:32 if that was the only way I could get the framing I wanted. The center is quite sharp at all apertures.
Mmm. Nice camera. But I doubt your sitters will find it very congenial having their portraits taken at f45. Even on HP5. Even in broad daylight.