I bought an old Edmunds Orthoscopic eyepiece, IIRC it was 12mm or 12.5mm. This was off the old, pre-Anacortes Astromart so it was a while ago. I used it in my f/5.8 refractor and while it was fine on axis, it was really, really bad off axis. Horribly bad.
I gave the eyepiece to a friend who had an f/8 scope and it worked better. I would not use these eyepieces in anything approaching f/6, better to stay at f/8 and above for them.