I don't think it's an artificial concept at all. It's a real characteristic of a specific eyepiece. When all distortions and aberrations are taken into account, the Hyperion 8mm will perform as a 76 degree eyepiece, and the UO Orthoscopic will perform as a 39 degree eyepiece.

Once that measurement is done, I can insert the eyepiece into a scope of a different focal length and predict what the true field will be.

Is that incorrect?