Though almost all the Mamiya RB lenses ("C" series and later) have the manually-adjustable floating elements which allow for highly corrected close-up images, the optical formula for all those lenses are optimised for subjects at infinity, i.e. parallel light rays entering the lens.
The 140mm Macro has an optical fomula that is optimised for closer subjects, i.e. light rays entering the lens at an angle. I am not sure what the optimal magnification is, but usually it's around 1:10 subject magnification or so.
That being said, the 140 appears to be absolutely phenominal at all distances I've used mine at (including landscape) but where it shines is perfect performance at close distances, even wide open (f/4.5). You will note that the Mamiya literature for other lenses, especially the wide angles, recommend stopping down to f/16 or more for decent performance. I frequently do 1:1 macro with mine, using both the Mamiya extension tubes together (makes for one darn large camera/lens comby, very unwieldy and difficult to use).
I have the "C" version, but unfortunately this lens (my copy, and others I've seen) seems very prone to separation of some of the glued-together glass elements, so check any copy that you buy carefully.
Words, however, cannot describe the performance of this lens - in my opinion. Pictures could, but I don't have a scanner (yet).
