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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.

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OK, I get what you're saying here. Not all eyepieces have a directly measurable field stop, however. So what the drift test gives me is a kind of "virtual field stop" number that I can use to predict true field of view for a given eyepiece in any telescope.

That seems rather arbitrary too, but at least I see how it can be tied to something physical -- even if it's just imaginarily so.

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For one eyepiece in particular, I have had to use this approach: the 5-8mm Speers Waler. Its variable effective focal length due to the changing location of the Smyth "field flattener" lens out in front meant that there was no way to actually measure any field stop. The only thing I could do was to measure the true fields at the ends of the focal length numbers and then work "backwards" to arrive at two "effective field stop" figures for the shortest and longest focal lengths of the eyepiece. However, this was far better than what the *company* actually did. They worked backward using the AFOV/Mag formula and got apparent fields that varied and were just plain silly to begin with (well beyond what the eyepiece actually provides). The apparent field of view of that eyepiece was actually fixed at a constant 79 degrees no matter how the eyepiece was set in focal length, so the company's claims were just plain ridiculous. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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