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George Huxtable wrote:
>
> 3. Only that part of the exit pencil that can pass through the pupil is
> collected by the eye to form an image. Any excess light goes to waste.

That's true. However, the wasted portion of the exit pupil gives your
eye latitude to move off-center while remaining fully illuminated.
That should be beneficial on a pitching deck.

With an oversized exit pupil, I believe the portion of objective lens
that's active depends on your eye's position. If it's exactly centered
in the exit pupil, the center of the objective is being used. Say your
pupil size is 5 mm and the scope is 4x40. Then the center 20 mm of the
objective is active. If you move your eye off-center in the exit
pupil, the active area of the objective moves too. This could be used
to vary the relative illumination of the horizon and body. That's what
intuition tells me, anyway. I own only bubble sextants, so I can't
test the theory.

There are times when an undersized exit pupil can help. Some astronomy
buffs report sharper stars with 10x50 binoculars compared to 7x50. The
smaller exit pupil avoids the outer area of the eye's lens, where
aberrations tend to be worst.

Also, as we age our maximum pupil opening tends to decrease. Many
people in middle age can attain only 5 mm in darkness, though in their
youth they could get 7 mm.

Some years ago I measured my maximum pupil size. The procedure
involves poking a pair of pinholes in a piece of metal foil and
holding it to your eye while viewing a suitable target at night. It
must be bright enough to be visible, but dim enough to keep your pupil
wide open. By trial and error you find the widest pinhole spacing that
fits your pupil.

I think the test can also be done by determining the widest opaque
strip you can see around on both sides. Same principle as the
pinholes, really.

   

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