[[1ponders] (Paul)]

To paraphrase from an exellent TV science show (and this is showing my age) "Why is this so?" I've worked out the Effective Field of all my eyepieces with various scopes I've have/had, using the formula you're suggesting. NP. But I've never questioned the reasoning behind the formula. And then the other night this annoying little voice in my head asked "Why doesn't the OTA aperture come into the figuring?" And you know how persistant that little voice can get.

I've done a bit of net surfing but haven't really come up with an acceptable answer. So I was hoping someone here could provide a simple, easy to understand answer. I accept that it doesn't and I am quite happy to work with the formula I know, but that damn little voice still wants to know "why doesn't it?"