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As I understand it there a a couple of other issues as
well.

You will all be familiar with the use of a pinhole as
a lens. In the case of night vision where the pupil is
substantially dilated it is unable to contribute this
"pin hole effect". This is compounded by presbyopia in
the ageing eye - we lose our ability to accommodate
because the lens hardens.

In addition night vision itself takes a good deal of
time to develop. A marine accident report in Australia
suggested that once night vision is impacted by being
subject to strong light it takes a "number of hours"
for it to return fully.

Finally there is the whole issue of not being able to
focus where there is nothing to focus on - on a black
night we have no point on which to accommodate and it
is not uncommon for our eye to therefore short focus
at some distance that appears neutral to it. For this
reason I always brief crew about the difficulty of
keeping watch for shipping at night and the need to
spend substantial time "working over" each sector
before concluding that it is "contact free". It's
amazing how often watch keepers miss a visual target.
You come on watch, scan the horizon and say "how long
has he been there?" and the watch keeper says "who".

I apologise for my lay person's explanation of this,
perhaps other list members may have a more
professional explanation.

Regards

Mike
--- Jared Sherman  wrote:

> Frank, I'm not sure that it is a "focus" issue per
> se. I have lasik damage, and during the day when my
> pupils are contracted I see better than I do at
> night. At night, glare, flare, and other distortion
> (like double vision and halos) become a problem
> because the cornea is now unevenly surfaced, and
> when the pupils are wide open at night--there is
> more lens area being used, so more distortions are
> in the "used" image path.
>
> Distortions in the cornea are actually common, and
> new procedures for corneal topography actually map
> the front and rear of the cornea separately to
> accomodate for them. Some of the ray tracing gizmos
> actually "project" a dot into your eye, and you use
> a joystick to mark a spot where you see it. That
> position is then compared to a mathematical position
> where a perfect eye SHOULD have seen the dot. And
> almost everyone has irregularities within the eye
> that show up during this kind of mapping.
>
> So with or without focus issues, your eyesight will
> be "degraded" when your pupils are wide open and
> more of the internal distortions affect the image
> you are seeing.


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