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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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Well, it will not be a number that corresponds to something which is physical, well understood, and can actually be measured. More importantly, it will not be the angle which your eye sees when it looks into the eyepiece (the definition of "apparent field of view"). It will be some number which, by jumping through various mathematical hoops, may or may not be able to accurately predict the true field of view. This is why I like the field stop. It is a physical dimension of the eyepiece that can be measured and that gives reasonably accurate values for the true field (within two percent, which is more than accurate enough). Tele Vue cites it in their eyepiece data for this very reason. I measured the field stops and apparent fields for all 12 of my eyepieces. The true fields of view predicted by the field stop formula all came within about 1.2 percent of the actual true field produced when the eyepieces were in the telescope. The apparent field of view figures for the eyepieces yielded AFOV/Mag results which were up to 6.6 percent off of the actual true field values. In general, the field stop formula seems to be, on average, up to five times more accurate at predicting the true field of view than the old AFOV/Magnification formula is. With the "generic" or inaccurate AFOV figures given by some retailers, the accuracy of the old formula can be even worse.

The field stop diameter very well *should* be the standard for eyepieces, although the apparent field of view is something people might like to look at to see how much of a span they will see when they look in the eyepiece. I see little reason to "calculate" some odd-ball "real apparent field of view" that pulls in the field stop number to the equation, when the simple field stop formula gives great results to begin with. Introducing some "fudged" apparent field is just unnecessary and potentially confusing. Clear skies to you.

Edited by David Knisely (05/18/08 03:15 AM)

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