In addition to accounting for the rectilinear distortion present within the eyepiece, the RFOV also corrects for the field of view modifying effects of the "real" focal length of an eyepiece as opposed to the focal length under which it is marketed (and upon which it is stamped). I believe this is why Baader's AFOV (actually RFOV) figures for the Eudiascopics are so far off from Orion's 52 degree AFOV for the Ultrascopics. The Eudiascopic (and Ultrascopic) eyepieces not only have inherent pincushion, but their focal lengths are also likely not as indicated (being in all instances somewhat less than indicated). This also could be a significant factor for the Hyperions TFOV vs. AFOV. Ditto for the Ethos. If in fact the Ethos exhibits noticeable pincushion of greater than about 1.7%, then its real focal length is likely not 13 mm.

People have often wondered why the pseudo-Masuyama's from Celestron (the original Ultima's) were sold as for example 12.5 mm, 18 mm, and 24 mm, whereas the others all sell them as 15 mm, 20 mm, and 25 mm respectively. Hmmm???