Acceptable deviation from axis value?

[liz]

My prescription for the left lens is:

-0.25 -3.25 173

When the glasses are made the left lens is actually:

-0.25 -3.25 175

Is that 2 degree difference for the axis within acceptable tolerance?
(I'm in the UK.)

[Dr. Big Blue Nation]

-0.25  -3.25  173
-0.25  -3.25  175
Is that 2 degree difference for the axis within acceptable tolerance?
(I'm in the UK.)

i live in the US. we have what are called ANSI standards for
ophthalmic lenses. I don't recall the exact numbers but, as an
optometrist, I can tell you that many people would "notice" 3.25
diopters of cylinder that is 2 degrees off-axis. The higher the
astigmatism correction (cylinder), the more precise the spectacles
must be made. my guess is that the tolerance is 1 degree with your
amount of astigmatism.

More importantly, with the proper tools, lenses can sometimes be
slightly rotated within the frame. Plus the frames can be adjusted
too. This could compensate for your lenses being slightly off by just
one or two degrees. A good experienced optician might be able to help
with the adjustments.

[r***@gmail.com]

I hope someone can help me and direct me. I am legally blind with limited Vision in only my left eye. I am trying to replace a broken pair of my glasses that I use for my distance vision. I recently went to an optometrist, had an exam and was given a pair of glasses, which I could not see well out of at all. The vision with my old broken was much better. I went back to the optometrist, and he said the only thing that he thought it might be due to was the astigmatism part of the script. He didn't really seem too sure so we both decided that since my old script had worked well for me we would just redo the new glasses the same as my old script. I did not have the written prescription so they took the script directly from my old glasses. The only difference between my older glasses and the new ones being made where that my old glasses were made from polycarbonate without an anti-glare coating and the new ones were going to be a high index glass with anti-glare coating. They said they can only redo the glasses one time and that if they didn't work out they would not be responsible. They assured me that the high index lens would give me even clearer Vision then the polycarbonate lens and the anti-glare coating would not affect the clarity or the vision. I went to pick up the new glasses today and once again the script did not seem correct! I can't focus as well with the new ones as my old ones even though they insist that the script is the same. I asked if it could be the way the lens was placed in the frame and they said it was placed accurately and the same as my old ones. They have refused to redo the lenses and will not give me my money back which I paid out of pocket without any insurance coverage. I immediately went down to another optometrist office and ask them to compare the script of my new glasses with my old ones. The OD on both prescriptions was the same,-300. I have no central vision in that right eye so it is mainly used as a balancing lens. The OS in my older glasses, which work for me, was -4.25-75-110. The other optician said that my OS script in my new glasses is-400-75x90. I do not know what any of these numbers mean. I asked them why the two numbers at the end were so different and how that would affect my vision. She told me that those numbers were they axis measurements for the astigmatism. With that mean that they are 20 degrees off? From reading the previous post I would think that 20° would make a big difference in the way I am focusing. I don't know if the measurements of-425 and-400 would make a big difference. I am trying to find out why I am having a problem getting a simple pair of single Vision distance glasses. These glasses were not sent out to a lab. They ordered the lens and put them in the frames themselves. Could it be possible they are not situated right in the frames or would the difference in the prescription be the problem? Since they are washing their hands of the problem is my only recourse to purchase another pair and throw away this money that I paid for these glasses? I am a disabled Widow and cannot afford another pair of glasses. Is there a society that over looks the optometrists that would assist me and to getting the correct pair of lenses from this optometrist? So now I am back to wearing my scratched glasses which have the arms held together with paper clips! All I wanted was the same exact lenses in the new frames! I am trying to understand what is causing the problem in the focusing and need advice on what I can do to rectify the situation. Please help!

[Joseph Gwinn]

Wrong newsgroup. Try<https://www.optometryforums.com>.

Post by r***@gmail.com
I hope someone can help me and direct me. I am legally blind with limited
Vision in only my left eye. I am trying to replace a broken pair of my
glasses that I use for my distance vision. I recently went to an optometrist,
had an exam and was given a pair of glasses, which I could not see well out
of at all. The vision with my old broken was much better. I went back to the
optometrist, and he said the only thing that he thought it might be due to
was the astigmatism part of the script. He didn't really seem too sure so we
both decided that since my old script had worked well for me we would just
redo the new glasses the same as my old script. I did not have the written
prescription so they took the script directly from my old glasses. The only
difference between my older glasses and the new ones being made where that my
old glasses were made from polycarbonate without an anti-glare coating and
the new ones were going to be a high index glass with anti-glare coating.
They said they can only redo the glasses one time and that if they didn't
work out they would not be responsible. They assured me that the high index
lens would give me even clearer Vision then the polycarbonate lens and the
anti-glare coating would not affect the clarity or the vision. I went to pick
up the new glasses today and once again the script did not seem correct! I
can't focus as well with the new ones as my old ones even though they insist
that the script is the same. I asked if it could be the way the lens was
placed in the frame and they said it was placed accurately and the same as my
old ones. They have refused to redo the lenses and will not give me my money
back which I paid out of pocket without any insurance coverage. I immediately
went down to another optometrist office and ask them to compare the script of
my new glasses with my old ones. The OD on both prescriptions was the
same,-300. I have no central vision in that right eye so it is mainly used as
a balancing lens. The OS in my older glasses, which work for me, was
-4.25-75-110. The other optician said that my OS script in my new glasses
is-400-75x90. I do not know what any of these numbers mean. I asked them why
the two numbers at the end were so different and how that would affect my
vision. She told me that those numbers were they axis measurements for the
astigmatism. With that mean that they are 20 degrees off? From reading the
previous post I would think that 20° would make a big difference in the way
I am focusing. I don't know if the measurements of-425 and-400 would make a
big difference. I am trying to find out why I am having a problem getting a
simple pair of single Vision distance glasses. These glasses were not sent
out to a lab. They ordered the lens and put them in the frames themselves.
Could it be possible they are not situated right in the frames or would the
difference in the prescription be the problem? Since they are washing their
hands of the problem is my only recourse to purchase another pair and throw
away this money that I paid for these glasses? I am a disabled Widow and
cannot afford another pair of glasses. Is there a society that over looks the
optometrists that would assist me and to getting the correct pair of lenses
from this optometrist? So now I am back to wearing my scratched glasses which
have the arms held together with paper clips! All I wanted was the same exact
lenses in the new frames! I am trying to understand what is causing the
problem in the focusing and need advice on what I can do to rectify the
situation. Please help!

[Mike Tyner]

Two degrees might exceed formal tolerance (dunno UK std). But opticians
correct for 2-degree rotation errors all the time.

2 degrees is 9:00:00 to 9:00:20 on the clockdial, like lifting your temple a
couple mm off your ear. Lenses can be rotated, frames can be bent.
Multifocals have their own constraints but tiny adjustments are easy.

Vision and comfort are more important than compliance.

If lenses are rotated badly, your focusing reflex recruits your oblique
muscles to cyclorotate. It's damn difficult to rotate one eye different from
the other so it's easy to make hi-cyl glasses uncomfortable. Sometimes the
true optical axis is different with wide pupils than it is in bright light.
Vertex distance and base curves make lenses weird too. Bottom line, if they
aren't clear and comfortable all day, adjustment is the first step. If
they're blurry now, or uncomfortable after a week, ask for a remake.

-MT, OD

Post by liz
-0.25 -3.25 173
-0.25 -3.25 175
Is that 2 degree difference for the axis within acceptable tolerance?
(I'm in the UK.)

[liz]

Thank you for the information. You say it is easy to make a 2 degree
rotation but the eyepiece of the frames is not very symmetrical. See this
photo: Loading Image...

Do you think a 2 degree correction is easily possible in those frames?

[I'm top posting like you to keep the quoted text in order.]

Post by Mike Tyner
Two degrees might exceed formal tolerance (dunno UK std). But
opticians correct for 2-degree rotation errors all the time.
2 degrees is 9:00:00 to 9:00:20 on the clockdial, like lifting your
temple a couple mm off your ear. Lenses can be rotated, frames can be
bent. Multifocals have their own constraints but tiny adjustments are
easy.
Vision and comfort are more important than compliance.
If lenses are rotated badly, your focusing reflex recruits your
oblique muscles to cyclorotate. It's damn difficult to rotate one eye
different from the other so it's easy to make hi-cyl glasses
uncomfortable. Sometimes the true optical axis is different with wide
pupils than it is in bright light. Vertex distance and base curves
make lenses weird too. Bottom line, if they aren't clear and
comfortable all day, adjustment is the first step. If they're blurry
now, or uncomfortable after a week, ask for a remake.
-MT, OD

Post by liz
-0.25 -3.25 173
-0.25 -3.25 175
Is that 2 degree difference for the axis within acceptable tolerance?
(I'm in the UK.)

[Dr. Big Blue Nation]

Post by liz
Thank you for the information. You say it is easy to make a 2 degree
rotation but the eyepiece of the frames is not very symmetrical. See this
photo:  http://i42.tinypic.com/suy69l.jpg
Do you think a 2 degree correction is easily possible in those frames?
[I'm top posting like you to keep the quoted text in order.]

Post by Mike Tyner
Two degrees might exceed formal tolerance (dunno UK std). But
opticians correct for 2-degree rotation errors all the time.
2 degrees is 9:00:00 to 9:00:20 on the clockdial, like lifting your
temple a couple mm off your ear. Lenses can be rotated, frames can be
bent. Multifocals have their own constraints but tiny adjustments are
easy.
Vision and comfort are more important than compliance.
If lenses are rotated badly, your focusing reflex recruits your
oblique muscles to cyclorotate. It's damn difficult to rotate one eye
different from the other so it's easy to make hi-cyl glasses
uncomfortable. Sometimes the true optical axis is different with wide
pupils than it is in bright light. Vertex distance and base curves
make lenses weird too. Bottom line, if they aren't clear and
comfortable all day, adjustment is the first step. If they're blurry
now, or uncomfortable after a week, ask for a remake.
-MT, OD

-0.25  -3.25  173
-0.25  -3.25  175
Is that 2 degree difference for the axis within acceptable tolerance?
(I'm in the UK.)

not the easiest frame to adjust cylinder axis (because of shape of
lens, double-bar bridge on frame) but its possible. if the difference
is truly just 2 degrees, then it is quite possible the adjustment
could be made by an experience optician or optical lab technician.
there is equipment that grinds and bevels plastic lenses after removal
from the frame, and its certainly possible to make slight adjustments
to the frame using the right tools that could also produce the
adjustment you need. Have you discussed this with your optometrist/
optician?

an important question that was brought up by Helmut in his reply above
relates to the question-- how accurate do you think the prescription
you have really is? a patient can readily tilt their head by more
that a couple of degrees and throw off the exam measurements? If
possible, I would remeasure again very carefully before you accept
that the prescription you have is really the gold standard.

also, measure the prescription off of current glasses that you have
that you think seem more comfortable and visually "normal".
sometimes, what I measure on a patient behind a phoropter (and verify)
doesn't product the best subjective results once the lenses are
mounted in frames. It could be a person becomes "adjusted" to their
habitual prescription even though it might not be exactly the same as
a doctor measures (in a dark room looking at a black and white chart--
not realistic). Likewise, you might adjust to the new glasses you
have that are just a couple of degrees off from what you are "used
to". just give it some time if you haven't already. patients with
high astigmatic corrections like you commonly have difficulty
adjusting to new glasses. but regardless, two degrees is doable.

[Salmon Egg]

In article

Post by Dr. Big Blue Nation
not the easiest frame to adjust cylinder axis (because of shape of
lens, double-bar bridge on frame) but its possible. if the difference
is truly just 2 degrees, then it is quite possible the adjustment
could be made by an experience optician or optical lab technician.
there is equipment that grinds and bevels plastic lenses after removal
from the frame, and its certainly possible to make slight adjustments
to the frame using the right tools that could also produce the
adjustment you need. Have you discussed this with your optometrist/
optician?
an important question that was brought up by Helmut in his reply above
relates to the question-- how accurate do you think the prescription
you have really is? a patient can readily tilt their head by more
that a couple of degrees and throw off the exam measurements? If
possible, I would remeasure again very carefully before you accept
that the prescription you have is really the gold standard.
also, measure the prescription off of current glasses that you have
that you think seem more comfortable and visually "normal".
sometimes, what I measure on a patient behind a phoropter (and verify)
doesn't product the best subjective results once the lenses are
mounted in frames. It could be a person becomes "adjusted" to their
habitual prescription even though it might not be exactly the same as
a doctor measures (in a dark room looking at a black and white chart--
not realistic). Likewise, you might adjust to the new glasses you
have that are just a couple of degrees off from what you are "used
to". just give it some time if you haven't already. patients with
high astigmatic corrections like you commonly have difficulty
adjusting to new glasses. but regardless, two degrees is doable.

There is an old adage about engineering solutions: Good enough is
perfect. How well do the glasses work as is? Is there any improvement in
vision if you rotate the glasses manually for the lens under
consideration? If a refraction for astigmatism is repeated several times
by a good practitioner, how repeatable is the orientation angle? That
presumes that the actual protractor is not observed until the refraction
is complete.

Finding the optimum angle is not easy. Best measurements are made by
measuring on the "skirts" of acuity rather than trying to find "best."

I think the principle easier to understand for polarization. French
scientists in the 19th century applied polarized light to measure sugar
content of grape juice. To get better accuracy, they used two analyzer
cocked slightly to each other. The angle was determined when they
perceived equal transmission from both analyzers. This "half-shade"
principle also applies to astigmatism orientation. That is why there is
a flipping in angle for astigmatism during the refraction procedure.

[liz]

Post by Salmon Egg
In article

Post by Dr. Big Blue Nation
not the easiest frame to adjust cylinder axis (because of shape of
lens, double-bar bridge on frame) but its possible. if the
difference is truly just 2 degrees, then it is quite possible the
adjustment could be made by an experience optician or optical lab
technician. there is equipment that grinds and bevels plastic lenses
after removal from the frame, and its certainly possible to make
slight adjustments to the frame using the right tools that could also
produce the adjustment you need. Have you discussed this with your
optometrist/ optician?
an important question that was brought up by Helmut in his reply
above relates to the question-- how accurate do you think the
prescription you have really is? a patient can readily tilt their
head by more that a couple of degrees and throw off the exam
measurements? If possible, I would remeasure again very carefully
before you accept that the prescription you have is really the gold
standard.
also, measure the prescription off of current glasses that you have
that you think seem more comfortable and visually "normal".
sometimes, what I measure on a patient behind a phoropter (and
verify) doesn't product the best subjective results once the lenses
are mounted in frames. It could be a person becomes "adjusted" to
their habitual prescription even though it might not be exactly the
same as a doctor measures (in a dark room looking at a black and
white chart-- not realistic). Likewise, you might adjust to the new
glasses you have that are just a couple of degrees off from what you
are "used to". just give it some time if you haven't already.
patients with high astigmatic corrections like you commonly have
difficulty adjusting to new glasses. but regardless, two degrees is
doable.

There is an old adage about engineering solutions: Good enough is
perfect. How well do the glasses work as is? Is there any improvement
in vision if you rotate the glasses manually for the lens under
consideration? If a refraction for astigmatism is repeated several
times by a good practitioner, how repeatable is the orientation angle?
That presumes that the actual protractor is not observed until the
refraction is complete.
Finding the optimum angle is not easy. Best measurements are made by
measuring on the "skirts" of acuity rather than trying to find
"best."
I think the principle easier to understand for polarization. French
scientists in the 19th century applied polarized light to measure
sugar content of grape juice. To get better accuracy, they used two
analyzer cocked slightly to each other. The angle was determined when
they perceived equal transmission from both analyzers. This
"half-shade" principle also applies to astigmatism orientation. That
is why there is a flipping in angle for astigmatism during the
refraction procedure.

Salmon Egg, thank you for your comments. Few of my glasses have felt
comfortable. I have partly ascribed the discomfort to neurological
problems I have in the occiptal region of the brain relating to vision and
balance. In other words, I may never obtain a comfortable result because
visual processing in my brain is not fully functioning.

However, I do want to reduce the discomfort as much possible and that's why
I originally posted to ask if the glasses were within specification because
if not then I would ask them to be corrected. I had wanted to know what
tolerance in the axis value is normally considered acceptable.

[liz]

Post by Dr. Big Blue Nation

Post by liz

Post by Mike Tyner

-0.25  -3.25  173
-0.25  -3.25  175
Is that 2 degree difference for the axis within acceptable
tolerance? (I'm in the UK.)

Two degrees might exceed formal tolerance (dunno UK std). But
opticians correct for 2-degree rotation errors all the time.
2 degrees is 9:00:00 to 9:00:20 on the clockdial, like lifting your
temple a couple mm off your ear. Lenses can be rotated, frames can
be bent. Multifocals have their own constraints but tiny
adjustments are easy.
Vision and comfort are more important than compliance.
If lenses are rotated badly, your focusing reflex recruits your
oblique muscles to cyclorotate. It's damn difficult to rotate one
eye different from the other so it's easy to make hi-cyl glasses
uncomfortable. Sometimes the true optical axis is different with
wide pupils than it is in bright light. Vertex distance and base
curves make lenses weird too. Bottom line, if they aren't clear and
comfortable all day, adjustment is the first step. If they're
blurry now, or uncomfortable after a week, ask for a remake.
-MT, OD

Thank you for the information. You say it is easy to make a 2 degree
rotation but the eyepiece of the frames is not very symmetrical. See
this photo:  http://i42.tinypic.com/suy69l.jpg
Do you think a 2 degree correction is easily possible in those frames?

not the easiest frame to adjust cylinder axis (because of shape of
lens, double-bar bridge on frame) but its possible. if the difference
is truly just 2 degrees, then it is quite possible the adjustment
could be made by an experience optician or optical lab technician.
there is equipment that grinds and bevels plastic lenses after removal
from the frame, and its certainly possible to make slight adjustments
to the frame using the right tools that could also produce the
adjustment you need. Have you discussed this with your optometrist/
optician?
an important question that was brought up by Helmut in his reply above
relates to the question-- how accurate do you think the prescription
you have really is? a patient can readily tilt their head by more
that a couple of degrees and throw off the exam measurements? If
possible, I would remeasure again very carefully before you accept
that the prescription you have is really the gold standard.
also, measure the prescription off of current glasses that you have
that you think seem more comfortable and visually "normal".
sometimes, what I measure on a patient behind a phoropter (and verify)
doesn't product the best subjective results once the lenses are
mounted in frames. It could be a person becomes "adjusted" to their
habitual prescription even though it might not be exactly the same as
a doctor measures (in a dark room looking at a black and white chart--
not realistic). Likewise, you might adjust to the new glasses you
have that are just a couple of degrees off from what you are "used
to". just give it some time if you haven't already. patients with
high astigmatic corrections like you commonly have difficulty
adjusting to new glasses. but regardless, two degrees is doable.

To answer your question about how accurate is the reading of the current
lenses .... The optician measured the lenses themselves and I saw their
notes. They mentioned that their automatic 'facimiter' couldn't easily
read the lens probably because of the high level of astigmatism, so they
carefully made the readings with a manual facimiter.

You say I might adjust to the new axis but Mike Tyner explained to me why
muscles in each eye might find it hard to adapt to axes that were not
within spec.

[Mike Tyner]

Post by liz
Do you think a 2 degree correction is easily possible in those frames?

Yes. A 2 degree correction is easily possible with most metal frames like
that.

But it's almost too small to measure. Why do you want to? Do you see with
them ok? Do they cause you any discomfort with long wear? Would you want the
frame to look funny or risk breakage just to get the axis "right?"

Your picture looks like the seg line is rotated about 4-5 degrees CCW, hard
to tell, but more than 2 degrees. The seg error is CCW, same as the axis
error. So fixing the axis by 2 degrees CW should make the seg lines more
level.

To do that, you could rotate the left lens clockwise with a lens wrench. You
could bend the lower bridge bar symmetrically, concave-down for the same
effect, or you could bend the bridge bars asymmetrically, like a backwards-N
between the eyewires.

These alter the appearance and design of the frames but 2 degrees is too
small to matter cosmetically.

Adjustments come bundled in the brick-and-mortar price.

Do you see with them ok? Do they cause you any discomfort with long wear?

-MT

[liz]

Post by Mike Tyner

Post by liz
Do you think a 2 degree correction is easily possible in those frames?

Yes. A 2 degree correction is easily possible with most metal frames
like that.
But it's almost too small to measure. Why do you want to? Do you see
with them ok? Do they cause you any discomfort with long wear? Would
you want the frame to look funny or risk breakage just to get the axis
"right?"

MT, to answer you point see the comments I have posted 20 minutes ago to
Salmon Egg.

Post by Mike Tyner
Your picture looks like the seg line is rotated about 4-5 degrees CCW,
hard to tell, but more than 2 degrees. The seg error is CCW, same as
the axis error. So fixing the axis by 2 degrees CW should make the seg
lines more level.

Whoa! Go slow! I'm not an optician. What's CCW? (I tried to Google
it but got this very technical info: http://goo.gl/R08j1)

Are you saying that the straight edge of the bifocal segment is not
horizontal and it should be?

Post by Mike Tyner
To do that, you could rotate the left lens clockwise with a lens
wrench. You could bend the lower bridge bar symmetrically,
concave-down for the same effect, or you could bend the bridge bars
asymmetrically, like a backwards-N between the eyewires.
These alter the appearance and design of the frames but 2 degrees is
too small to matter cosmetically.
Adjustments come bundled in the brick-and-mortar price.

I was thinking that if the lens was sufficiently wrong then they might
replace it.

Post by Mike Tyner
Do you see with them ok? Do they cause you any discomfort with long wear?

They are not comfortable but as I was saying to Salmon Egg, few specs I
get are comfortable, because of some visual/balance neurological problem
which is being investigated. To make it even harder to assess comfort,
my neurological functioning fluctuates from hour to hour so confort is
very hard to assess.

If you want to know more, then the docs think it may be similar to Acute
Confusional States (or Clinical Delirium) which is a condition that
usually originates from a physical illness and results in depleted
choline & excess dopamine in the brain giving rise to fluctuating
problems with attention and level of consciousness. In my case, the
confued states often coincides with visual/balance problems. I have a
neurology professor trying to work out what's going on.

As usual in patients experiencing Acute Confusional States, senses are
impaired and one gets over-sensistive to ordinary stimuli including
visual. Last year visual movement was difficult and painful to process
as I was experiencing severe bouts of Visual Vertigo ("Supermarket
Syndrome").

Here's more info than you might want to know about this! .....
Delirium
http://goo.gl/Kljom
Visual Vertigo
http://brain.oxfordjournals.org/content/124/8/1646.full

[Neil Brooks]

Liz-

CW = clockwise
CCW = counter clockwise

I have a verrrrry complex set of eyes, and ... some sort of issue with
how my brain processes vision, too.

At nearly 50 years of age, typically, accommodation wouldn't play a
significant role.

But ... in my case ... it does.

You have a moderately high Rx. If you're even under 70 years old, I
might suggest that you talk with your eye doc about a "strong"
cycloplegic (dilating drops) eye exam, and refraction (determining
your prescription), using ... maybe Homatropine.

Since you're VERY sensitive to refractive error, it may allow a really
competent OD or MD to understand if you have uncorrected refractive
error, where ... adding additional correction/fine-tuning your
prescription ... could make you more comfortable.

Good luck !

[Mike Tyner]

Post by Neil Brooks
You have a moderately high Rx. If you're even under 70 years old, I
might suggest that you talk with your eye doc about a "strong"
cycloplegic (dilating drops) eye exam, and refraction (determining
your prescription), using ... maybe Homatropine.

I disagree, if the goal is to make a comfortable pair of glasses.

Cycloplegic refraction is useful in kids with vigorous accommodation, but in
presbyopes, dilating the pupil adds more variability than cycloplegia
removes. Glasses made with homatropine might only work when taking
homatropine.

-MT

[Neil Brooks]

Post by Mike Tyner

You have a moderately high Rx.  If you're even under 70 years old, I
might suggest that you talk with your eye doc about a "strong"
cycloplegic (dilating drops) eye exam, and refraction (determining
your prescription), using ... maybe Homatropine.

I disagree, if the goal is to make a comfortable pair of glasses.
Cycloplegic refraction is useful in kids with vigorous accommodation, but in
presbyopes, dilating the pupil adds more variability than cycloplegia
removes. Glasses made with homatropine might only work when taking
homatropine.
-MT

Fair enough.

Liz -- to me -- sounded ophthalmogically complicated.

My thought was more along the lines of knocking out another -
potential- (if not very likely) variable.

[Mike Tyner]

Post by liz
Are you saying that the straight edge of the bifocal segment is not
horizontal and it should be?

Yes, it's sorta standard for the lines on each side to be parallel with each
other, though sometimes not at the same level. When they aren't parallel,
one lens has been rotated and the rotation is CCW for counterclockwise in
your photo.

I couldn't be sure your segs aren't parallel. I need a photo that shows both
lenses straight-on, from a little lower than your nose. From what I see here
the left lens seg line looked rotated CCW. The Rx was also off CCW. Fixing
both with 2 degrees of rotation could make the glasses more comfortable.
Knowing now that you have a local optitian, SURE, take them back and ask
them to adjust your axis. Don't worry about the seg line so much.

Post by liz
They are not comfortable but as I was saying to Salmon Egg, few specs I
get are comfortable, because of some visual/balance neurological problem
which is being investigated. To make it even harder to assess comfort,
my neurological functioning fluctuates from hour to hour so confort is
very hard to assess.

And I did say that 2 degrees is a tiny error.

Post by liz
If you want to know more, then the docs think it may be similar to Acute
Confusional States (or Clinical Delirium) which is a condition that
usually originates from a physical illness and results in depleted
choline & excess dopamine in the brain

Most of the binocular reflexes involved in vision occur in the midbrain
using some very primitive input from visual cortex. Deeper CNS difficulties,
psychopathology and altered states are way over my pay grade.

[Salmon Egg]

Post by liz
-0.25 -3.25 173
-0.25 -3.25 175
Is that 2 degree difference for the axis within acceptable tolerance?
(I'm in the UK.)

I AM NOT AN OPTOMETRIST OR OTHER MEDICAL VISION PROFESSIONAL!
Nevertheless, I do not think that the difference you describe is very
significant. Just how you put on your glasses could cause that much of
an error.

[Helmut Wabnig]

Post by liz
-0.25 -3.25 173
-0.25 -3.25 175
Is that 2 degree difference for the axis within acceptable tolerance?
(I'm in the UK.)

Lens frames bend more than that.
A short breeze of wind (so to say) will move the whole thing
far more than your numbers. Noses too have tolerances.

The question is (you did not mention)
how you measured those numbers and to what accuracy.


Summa summarum I say: forget it.


w.

[Robert Martellaro]

Post by liz
-0.25 -3.25 173
-0.25 -3.25 175
Is that 2 degree difference for the axis within acceptable tolerance?
(I'm in the UK.)

Ansi standards are .13 D (half of the power that most people can see as a just_
noticeable_ difference in vision) for visual tolerance, and 2 deg. for
production and mechanical alignment for cylinder powers above 1.50 D .

The axis tolerance = arcsin [cyl tolerance / (2 * cyl power)].

For your Rx, arcsin [.13 / 2 * 3.25] = 1.15 deg.

As others have mentioned, I would turn the lens so that it is ±1 deg.

http://i42.tinypic.com/suy69l.jpg

I have some concerns about the segment height placement. The very low position
(10mm high?) might be satisfactory for someone who reads very little, but only
if the vertical optical centers have been positioned properly (most labs will
place the OCs about 3mm above the seg), or if the pantoscopic angle is
increased. Failure to do so will increase the cylinder power by about .25 D.
There are additional concerns with perspective changes and discomfort due to the
yoked base down prism, if the OCs are unusually low, relative to the pupil.
Your optician should check the vertical OC position; The pantoscopic angle
should be 2 deg for every 1mm the OC is below the pupil.

I would also recommend an atoric lens due to the high degree of astigmatism.
There's potential for slightly clearer vision off-axis, with most folks feeling
a sense of increased refinement overall. Hoya's Straight Top 28 IQ should be
available in most countries.

Hope this helps,

Robert Martellaro
~~~~~~~~~~~~~~~~~~
Roberts Optical Ltd.
Wauwatosa Wi.
www.roberts-optical.com
~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman

[liz]

Post by Robert Martellaro

Post by liz
-0.25 -3.25 173
-0.25 -3.25 175
Is that 2 degree difference for the axis within acceptable tolerance?
(I'm in the UK.)

Ansi standards are .13 D (half of the power that most people can see
as a just_ noticeable_ difference in vision) for visual tolerance, and
2 deg. for production and mechanical alignment for cylinder powers
above 1.50 D .
The axis tolerance = arcsin [cyl tolerance / (2 * cyl power)].
For your Rx, arcsin [.13 / 2 * 3.25] = 1.15 deg.
As others have mentioned, I would turn the lens so that it is ±1 deg.
http://i42.tinypic.com/suy69l.jpg
I have some concerns about the segment height placement. The very low
position (10mm high?) might be satisfactory for someone who reads very
little, but only if the vertical optical centers have been positioned
properly (most labs will place the OCs about 3mm above the seg), or if
the pantoscopic angle is increased. Failure to do so will increase the
cylinder power by about .25 D.

Thank you for the useful feedback Robert. I bought the bifocal specs to
use when shopping, so they combine far vision with near vision (+1.50
D). I get surprisingly dizzy in a supermarket even with single vision
lenses, so we decided on a frame with a large eyepiece and the optician
suggested a very low close up segment. Both factors were chosen to
minimise peripheral uncorrected vision.

I believe the optician checked and tilted the pantoscopic angle of the
frames quite a lot.

Post by Robert Martellaro
There are additional concerns with perspective changes and discomfort
due to the yoked base down prism, if the OCs are unusually low,
relative to the pupil.

I don't know anything about yoked prisms but after your post I read they
are used to "move the world in one direction". Are you saying you can
tell from my photo that I have got yoked prisms or are you saying that I
should consider them?

Post by Robert Martellaro
Your optician should check the vertical OC position; The pantoscopic
angle should be 2 deg for every 1mm the OC is below the pupil.
I would also recommend an atoric lens due to the high degree of
astigmatism. There's potential for slightly clearer vision off-axis,
with most folks feeling a sense of increased refinement overall.
Hoya's Straight Top 28 IQ should be available in most countries.
Hope this helps,
Robert Martellaro

[Robert Martellaro]

Post by liz
Thank you for the useful feedback Robert. I bought the bifocal specs to
use when shopping, so they combine far vision with near vision (+1.50
D).

The Add power is +1.50? Do you wear your glasses full time? Do you read or
perform general close tasks with the eyeglasses off?

Post by liz
I get surprisingly dizzy in a supermarket even with single vision
lenses, so we decided on a frame with a large eyepiece and the optician
suggested a very low close up segment. Both factors were chosen to
minimise peripheral uncorrected vision.

Generally, smaller and closer (to the eye) minimizes off-axis aberrations and
distortion. The segment is usually level with the lower limbus or eyelid, but
might be set slightly lower in some instances, for instance, very little close
tasks, or when they are removed when reading.

Post by liz
I don't know anything about yoked prisms but after your post I read they
are used to "move the world in one direction". Are you saying you can
tell from my photo that I have got yoked prisms or are you saying that I
should consider them?

The prism is induced by placing the vertical optical center at some other point
besides center pupil. I'm concerned that the unusually lower segment position
may have lowered the OC too far. The OC should be about 3mm t0 5mm below the
pupil. More than three diopters or so of yoked prism can cause some discomfort.

BTW, I got it wrong- the prism induced with your Rx, if the OC is below the
pupil, is base up, not base down.

If you can not wear the eyeglasses without feeling dizzy, even SV as you stated
above, and the lenses are optimally designed and positioned, one might ask the
doctor to reduce the astigmatic power somewhat. This may increase the
wearability, but will decrease the visual acuity slightly. This can be
demonstrated in the chair, or better still with a trial frame that will allow
you to walk around to better evaluate performance.

Robert Martellaro
~~~~~~~~~~~~~~~~~~
Roberts Optical Ltd.
Wauwatosa Wi.
www.roberts-optical.com
~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman