optical illusion or atmospheric effect?

On 25/05/2012 15:18, Yousuf Khan wrote:
On 25/05/2012 3:06 AM, Martin Brown wrote:

With your glasses on your long distance vision is over corrected at
infinity and without them it is under corrected. The result is you see
each star as two spots or a line : or ..

ITYM 3-5 arc minutes separation too.

Interesting, describes almost exactly what I saw.

I wonder if it's worth getting a new prescription done, based on this
situation? Or will getting a prescription corrected on watching stars
make the glasses useless for regular use?

Probably not a good idea. I have just reached the stage where I can tell
my uncorrected night vision is no longer tack sharp at infinity. It
isn't a problem but it does affect the limiting magnitude.

Correcting for the worst case maximum aperture at one extreme of
distance isn't likely to be helpful in ordinary every day use.

--
Regards,
Martin Brown

William Hamblen wrote:
On 5/25/2012 10:34 AM, Mike Dworetsky wrote:
William Hamblen wrote:
On 5/25/2012 3:49 AM, Mike Dworetsky wrote:

I'll agree with the other responses that this is astigmatism in
your eye, and add that spectacle corrections are not always
perfect, which is why you still saw some residual astigmatism in
the perpendicular direction. I once got prescription lenses that
actually made vision in one eye worse, and was told that, "Well,
the angle tolerance allowed in the workshop is 20 degrees +/-, good
enough for most people." I told the optician to send it back and
get a new one made, which was spot on, and gave complete
correction, at their expense.

That optical shop was lying to you. The angle tolerance in cylinder
is supposed to be only TWO degrees, not twenty, depending on the
amount of cylinder in the prescription.

Thanks. It was the optician's assistant who said this to me, not the
lens maker, who is located a long way from London.

Well, you may be right. Though judging 2 degrees from looking at
letters and spots, and listening to the patient saying, "Maybe this
one is a bit better than the last one," and adjusting the angles on
test lenses during an exam, seems to be expecting a lot from the
optician. OTOH if you say the tolerance allowed for the lens maker
is 2 degrees variation from the prescription, this makes more sense.

I'll check it out. Since changing opticians a decade ago (after
moving to a new address) I haven't had any further problems with
these prescriptions.


The tolerance, of course, is in the lens fabrication, which is why the
statement about plus or minus twenty degrees _in the workshop_ was
bunk. Clinical error is another thing altogether, but a careful eye
doctor and a willing patient should be able to get the angle closer
than plus or minus 20 degrees. Your correction isn't ever 100%
stable, either. You need to get it checked periodically. I myself

Certainly true, I have a check once a year.

have irregular astigmatism in my right eye that makes it impossible
to reach an end-point correction in the eye doctor's chair. We go
until we get tired of trying. Roughly 20/25 (6/7.5) is about the
best acuity I've ever had in that eye. The left eye, although it
takes a stronger prescription in both sphere and cylinder, will be
20/20 (6/6) with the right prescription.

Mine does not vary so much, but from time to time a new prescription is
necessary. Which I hate, because the blended lenses are really expensive...

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

On Fri, 25 May 2012 17:53:33 +0100, Martin Brown
wrote:
On 25/05/2012 15:18, Yousuf Khan wrote:
On 25/05/2012 3:06 AM, Martin Brown wrote:

With your glasses on your long distance vision is over corrected
at
infinity and without them it is under corrected. The result is
you see
each star as two spots or a line : or ..

ITYM 3-5 arc minutes separation too.

Interesting, describes almost exactly what I saw.

I wonder if it's worth getting a new prescription done, based on
this
situation? Or will getting a prescription corrected on watching
stars
make the glasses useless for regular use?

Probably not a good idea. I have just reached the stage where I can
tell
my uncorrected night vision is no longer tack sharp at infinity. It
isn't a problem but it does affect the limiting magnitude.

Correcting for the worst case maximum aperture at one extreme of
distance isn't likely to be helpful in ordinary every day use.

--
Regards,
Martin Brown

One can get two pairs of glasses, one for everyday use and another
for astronomical use.

On 25/05/2012 21:03, Paul Schlyter wrote:
On Fri, 25 May 2012 17:53:33 +0100, Martin Brown
wrote:
On 25/05/2012 15:18, Yousuf Khan wrote:

I wonder if it's worth getting a new prescription done, based on
this
situation? Or will getting a prescription corrected on watching
stars
make the glasses useless for regular use?

Probably not a good idea. I have just reached the stage where I can
tell
my uncorrected night vision is no longer tack sharp at infinity. It
isn't a problem but it does affect the limiting magnitude.

Correcting for the worst case maximum aperture at one extreme of
distance isn't likely to be helpful in ordinary every day use.


One can get two pairs of glasses, one for everyday use and another for
astronomical use.

I am planning on doing just that if it gets any worse. For now I can
still see enough of the constellations ~mag 4.5 to do public stargazing
talks. I have a tame optician friend in the village but it is finding
time to do an eye test when dark adapted and he has brought his kit
home. I estimate my far point is now about 30m instead of infinity, and
my near point has now receded well beyond the length of my arms.

In daylight +0.25 diopter is enough to bring infinity tack sharp again
(and also in the classic opticians test where infinity is taken to be
about 30' by convention). But dark adapted with a fully dilated pupil I
estimate my correction should be about -0.25 - no amount of positive
correction seems to help (easily tested with reading glasses).

--
Regards,
Martin Brown