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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?

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No, the Hyperion's apparent field will not be 76 degrees, i.e. the eye will not see an apparent field that subtends 76 degrees. Your true field measurement cannot be backed into apparent field without taking distortion into account.
The apparent field is probably the 68 degrees claimed, and the true field is what you measured. The compensatory factor required to reconcile the two is distortion.
An example of how timed passage of stars cannot derive an apparent field, for exaggeration purposes, is a sphere.
Let's say you're in space, watching the Earth turn. You time the passage of a city as it passes from one "edge" to the other. The timed passage (12 hrs) indicates the width of the Earth is wider than you see it. How? Well, during a lot of the time you watch the passage, the city is not moving directly sideways relative to your eye but away and toward your eye.
It moves slowly when it first appears, moves quickly as it passes the center of the globe, and then slows down again.

The passage of a star across the field of an eyepiece is a lot like that. It doesn't move at a uniform speed, but changes according to the amount of distortion. Thus, timing a star will give you the true field, but it will not give you the apparent field unless you know the distortion factor.

The way I look at it, true field is one thing, apparent field is another. And there's no easy way to derive one from the other.

DonP

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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member

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