Re: Seeing is Believing
- From: "Ioannis" <morpheus@xxxxxxxxxxxx>
- Date: Thu, 11 Aug 2005 20:55:56 +0300
Ο <tony_flanders@xxxxxxxxx> έγραψε στο μήνυμα
news:1123724519.335414.301210@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
[snip for brevity]
> A couple of people sent me E-mails suggesting that this
> was a phenomenon with a name: night myopia. I heard that
> optometrists had been quietly prescribing different
> glasses for night truck drivers for years. And Chris
> Peterson suggested that I could test the hypothesis at
> zero cost my tilting my glasses 45 degrees, increasing
> their diopter rating at the cost of introducing astigmatism.
> The experiment worked -- I could definitely see more
> stars than usual through my tilted glasses.
[snip]
Ok, this is a good article. A couple of minor points:
The "slightly undercorrected" eyeglasses is standard procedure among
opthalmologists. The reason is simple: The combination of human eye/brain is
"lazy". That is, given a perfectly corrected lens, the myopic eye has the
tendency to become myopic again, for reasons unknown.
Opthalmologists usually prescribe slightly undercorrected lenses, in order
to keep the system in the realm of "always trying" to focus better.
Given a perfectly corrected lens, the eye/brain will adjust quickly to the
higher resolution and clarity and then there is no going back to the
slightly undercorrected lens.
Also, there is, as you imply, a certain difference in what is meant by
"perfectly corrected", for daytime and nightime use. A perfectly corrected
night time prescription may turn to be uncomfortable during the day. A
perfectly corrected day time prescription may turn out to be insufficient
during night time.
Since eye measurements are done in eye labs usually under conditions of
medium ambient light, the usual daytime prescription will always be slightly
undercorrected for night vision (when the eye pupil enlarges and introduces
some small spherical abberation) and it will be slightly overcorrected
during intense sunlight (when accuity increases because of the increased f
number)
A simple way to slightly overdrive your lenses is to not tilt, rather
"raise" the lens. Tilting introduces more astigmatism than "raising".
Raising the lens, one introduces "prism", which in effect causes the eye to
look through a thicker part of the lens. Since raising the lens does not
cause rotation of the optical axis, the astigmatism is less than with
tilting, which rotates the optical axis.
The best solution is the one that you came to conclude: Have two
prescriptions, one for daytime use, (the usual one) and one slightly
overcorrected, for nighttime use.
A note of caution applies: If you spend too much time viewing with the
overcorrected prescription, your eye will become "lazy" again and it will
require constant usage of the overcorrected prescription.
A good alternative to a double prescription is the following procedure,
which I have tried successfully and is absolutely safe:
Remove your eye glasses and keep them off for 2-3 hours prior to an
observing session. After 2-3 hours in normal lighting conditions, the
eye/brain system starts adjusting towards the "without glasses" case, so it
will make an effort to "optimise" (as much as it possibly can) its focusing
mechanism. Although still faulty, the optimized condition will ensure than
when you put your glasses back on, there will be a delayed residual effect
which will cause a temporary sharper vision for a duration of approximately
half an hour to an hour until the eye/brain system starts adjusting towards
the "with glasses" case, depending on circumstances.
> Why would you expect anything different? Human vision
> is immensely variable, immensely complicated, and
> still very poorly understood.
Very very true :-)
> - Tony Flanders
--
I. N. Galidakis
http://users.forthnet.gr/ath/jgal/
Eventually, _everything_ is understandable
.
- References:
- Seeing is Believing
- From: tony_flanders
- Seeing is Believing
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