Originally posted by pid I have the REF-M. At 1x magnification you can read the digital readout completely. At the 2x magnification you only can read the middle of the readout, but this doesn't matter, because you can easy switch between the two magnifications, if you want to see the whole line. I think it is no big issue. It works like a charm on my K-5
Thank you, Pid, for answering the question on how well the Ref-M works with current DSLRs, and that's good news.
AFAIK, Pentax produced 4 "Refconverters" (as Pentax called its right-angle finder accessories), and I do believe the following:
1. The original "Refconverter" fits all Pentax bodies, including most screwmount bodies, up through the K series (KX, K2, K1000, KM), but it provided a left-right reversed mirror image.
2. The "Refconverter-II" fits the same bodies as above, but it provided proper left-right imaging.
3. The "Refconverter-M" fits all Pentax bodies starting with the M series (ME, MX, ME Super, etc.) (with proper left-right imaging), and provides a 1X/2X switch and the option of having 1X or 2X magnification. (The M bodies introduced a slightly different sized flange around the viewfinder window, requiring the new Refconverter coupling.)
4. The "Refconverter-A", which came out with the arrival of the first A bodies (the Super Program in North America and the Super A elsewhere), is exactly like the "Refconverter-M" except that it had a taller aperture, making it easier to see the LCD digital readout display at the bottom of the Super Program/A viewfinder windows. (My hunch is that the Ref-M and Ref-A might be optically identical, and the only difference might be the taller "cut-out" on the coupling face in the Ref-A.)
Oh, and the Ref-M and Ref-A are also a bit trimmer in size and shape than the original Refconverters.
There are a bunch of reviews at
https://www.pentaxforums.com/accessoryreviews/pentax-refconverter-a.html