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John Karl writes-

| Simple ray tracing (as I discuss in my book) shows why a lens using a
| focal plane can image the entire field even though half of the
| objective is blocked off.  This analysis is confirmed with all scopes
| that I have seen that use an internal focal plane.
|
| But using the same simple ray tracing, I can't exactly see why the
| Galilean scope (no eternal focal plane) doesn't display the whole
| field also.  I think it's related to vignetting, depth of field, and
| practical optical engineering, not something explained by simple ray
| tracing.  Nonetheless, experimentally, every Galilean scope I've seen
| shows only half the FOV when the other half is blocked off at the
| objective.
|
| So I understand why prism scopes show the entire field, but not why
| Galilean scopes don't.  In my book I explain the former and ignore the
| latter -- you don't expect an author to confess his ignorance, do you?

I like the honesty in those final words.

John words- "I think it's related to vignetting, depth of field, and
practical optical engineering, not something explained by simple ray
tracing." seem somewhat obscurantist to me. I have suggested, just as a
result of observation, and without any serious technical argument to back
it, that it could simply be a function of the magnification, but he appears
to ignore that notion. Why?

To make some sort of test we need two differently-contrived scopes; a
prismatic and a Galilean, with about the same level of magnification, to
compare. Unfortunately, there's no overlap between those that I have here.
The Galileans are only x3, the others x6 or more. Perhaps that's the general
case.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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