My Eyeglasses May Have Wrong Base Curve! Need Advice!
midwe...@yahoo.com
Prior to 1997, I wore glasses that were -4.25 in the left eye and -4
in the right eye. These glasses had a base curve of 3.5 in each lens.
Also, the lenses were very big in size. Also, the lens material was
glass (I believe).
>From 1997 to 2004, I wore -5, -5 glasses for driving. I continued to
use my -4.25, -4 glasses as reading glasses - for reading, computer
use, etc. The -5 glasses also had big lenses (about the same size as
the lenses in the -4.25, -4 glasses). Unfortunately, I never had the
base curve of these glasses measured, and I lost them in 2006. Also, I
am uncertain as to the lens material of the -5 glasses.
>From 2004 to the present day, I have used -5.25, -5.25 glasses for
driving. I continued to use the -4.25, -4 glasses for reading,
computers, etc. The base curve of the -5.25 glasses is 2.5 in each
lens. However, the -5.25 glasses have lenses that are significantly
smaller than the lenses in my -4.25, -4 glasses. Also, the -5.25
glasses have lenses made of photo flextint plastic.
By the way, the base curve of the -4.25, -4 glasses and the base curve
of the -5.25 glasses were measured at my eye doctor's office.
Recently, I decided to get some reading glasses with small lenses. So,
I ordered a pair of glasses that had the following characteristics:
prescription: -4.25, -4.25 (approximately the same as the prescription
of my previous reading glasses)
base curve: 3.5 in each lens
size of lenses: slightly smaller than the -5.25 lenses (significantly
smaller than the lenses in the -4.25, -4 glasses)
lens material: photo flextint plastic
The new reading glasses have approximately the same prescription as my
old reading glasses. The new glasses have the same base curve as my
old glasses.
However, the new glasses are made of photo flextint plastic, and the
old glasses are made of glass.
Also, the new glasses have significantly smaller lenses than the old
glasses.
Well, the new glasses give me headaches, while the old glasses do not
give me headaches. So, what is causing these headaches?
I bought my new glasses at Lenscrafters. Is it possible that my doctor
and Lenscrafters measure the base curve in two different ways?
My doctor measured the base curve of the old glasses at 3.5. When I
asked Lenscrafters for a base curve of 3.5, perhaps Lenscrafters
produced lenses that Lenscrafters measured at 3.5 but that my doctor
would measure as something else?
Or, since the lenses of the old glasses are big and have a base curve
of 3.5, perhaps the base curve of smaller lenses of approximately the
same prescription should be a different base curve?
Or is the difference in lens material causing the headaches? These new
glasses are the first glasses I've had that have a prescription in the
-4.25 range and that are made of photo flextint plastic. Prior to now,
the glasses that I had in the -4.25 range were made of glass.
So, please give me some advice. Thanks for any information.
michael toulch
frame fit, optical centers may play a role. base curve may have
nothing to do with it - have an optician at lens crafters compare both
pairs.
midwe...@yahoo.com
> frame fit, optical centers may play a role. base curve may have
> nothing to do with it - have an optician at lens crafters compare both
>
I went to Lenscrafters and had the optical centers checked out.
The optical centers on the new reading glasses are almost at the
centers of my eyes. So, the new reading glasses are OK as far as the
optical centers are concerned. However, the old reading glasses'
optical centers are below the centers of the eyes (by approximately 4
to 5 mm).
The optician also said that the new lenses are smaller than the old
lenses and that I'm noticing the bottom of the new lenses whereas I
had not noticed the bottom of the old lenses. In other words, I would
have to get used to the new glasses.
However, I then had the optician measure the base curve of all three
pairs of glasses (-5.25 glasses, old reading glasses, and new reading
glasses). The optician measured the base curve using something called
a lens clock.
Anyway, he measured the base curve of the -5.25 glasses at 2.5, which
confirms my doctor's measurement. The optician measured the base curve
of the new reading glasses at 3.5, which is what I had ordered.
However, the optician measured the base curve of the old reading
glasses at 3.0, while my doctor's measurement is at 3.5. So, either my
doctor or the optician made an incorrect measurement.
Since the new glasses give me a headache and the old ones don't, I'm
guessing that the 3.0 measurement on the old ones is correct and that
I should exchange the new glasses for glasses that have a base curve
of 3.0.
Robert Martellaro
>On Aug 17, 8:02?pm, michael toulch >
>> frame fit, optical centers may play a role. base curve may have
>> nothing to do with it - have an optician at lens crafters compare both
>> pairs.- Hide quoted text -
>>
>
>
>I went to Lenscrafters and had the optical centers checked out.
>
>The optical centers on the new reading glasses are almost at the
>centers of my eyes. So, the new reading glasses are OK as far as the
>optical centers are concerned. However, the old reading glasses'
>optical centers are below the centers of the eyes (by approximately 4
>to 5 mm).
The OC should be about 4mm to 5mm below the pupil, along with about 10 degrees
of panto (lens tilt). This aligns the optical axis of the lens with the center
of rotation of the eye. Failure to do so will induce power and astigmatic error
on and off-axis. I would also use the distance instead of the near PD for the
reading glasses.
It sounds like the frame is sitting too high on the bridge, although the
vertical OC can be placed lower if needed. I would prefer a frame that fits in a
way that places the vertical center below the pupil.
>The optician also said that the new lenses are smaller than the old
>lenses and that I'm noticing the bottom of the new lenses whereas I
>had not noticed the bottom of the old lenses. In other words, I would
>have to get used to the new glasses.
That doesn't make sense, using a narrow frame that sits high for the reading
glasses. I'm sure the frames look very nice on you though.
>However, I then had the optician measure the base curve of all three
>pairs of glasses (-5.25 glasses, old reading glasses, and new reading
>glasses). The optician measured the base curve using something called
>a lens clock.
Lens clocks are calibrated for crown glass, and will give inaccurate readings on
materials with different index of refraction, unless you modify the reading to
account for this. Lens clocks will always give inaccurate results with aspheric
surfaces.
>Anyway, he measured the base curve of the -5.25 glasses at 2.5, which
>confirms my doctor's measurement. The optician measured the base curve
>of the new reading glasses at 3.5, which is what I had ordered.
>However, the optician measured the base curve of the old reading
>glasses at 3.0, while my doctor's measurement is at 3.5. So, either my
>doctor or the optician made an incorrect measurement.
There is only one (technically there's two, but the other one is usually not
practical for most ophthalmic applications) correct base curve per Rx, lens
material, and surface design. With few exceptions, the base curve should be
determined by the manufacturer of the lens blank.
>Since the new glasses give me a headache and the old ones don't, I'm
>guessing that the 3.0 measurement on the old ones is correct and that
>I should exchange the new glasses for glasses that have a base curve
>of 3.0.
That's not the problem. Assuming there are no other issues with the lens
fabrication, and that the reading glasses were prescribed by an eye doctor, I
would concentrate on the OC position and frame selection. It would probably be
best to avoid polycarbonate to rule out any sensitivity to chromatic aberration.
Hope this helps.
Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical
Wauwatosa Wi.
~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman
michael toulch
> On Aug 17, 8:02?pm, michael toulch >
>
> > frame fit, optical centers may play a role. base curve may have
> > nothing to do with it - have an optician at lens crafters compare both
> > pairs.- Hide quoted text -
>
> I went to Lenscrafters and had the optical centers checked out.
>
> The optical centers on the new reading glasses are almost at the
> centers of my eyes. So, the new reading glasses are OK as far as the
> optical centers are concerned. However, the old reading glasses'
> optical centers are below the centers of the eyes (by approximately 4
> to 5 mm).
**right there - that is a significant difference -the o.c. height.
>
> The optician also said that the new lenses are smaller than the old
> lenses and that I'm noticing the bottom of the new lenses whereas I
> had not noticed the bottom of the old lenses. In other words, I would
> have to get used to the new glasses.
>
> However, I then had the optician measure the base curve of all three
> pairs of glasses (-5.25 glasses, old reading glasses, and new reading
> glasses). The optician measured the base curve using something called
> a lens clock.
>
> Anyway, he measured the base curve of the -5.25 glasses at 2.5, which
> confirms my doctor's measurement. The optician measured the base curve
> of the new reading glasses at 3.5, which is what I had ordered.
> However, the optician measured the base curve of the old reading
> glasses at 3.0, while my doctor's measurement is at 3.5. So, either my
> doctor or the optician made an incorrect measurement.
>
> Since the new glasses give me a headache and the old ones don't, I'm
> guessing that the 3.0 measurement on the old ones is correct and that
> I should exchange the new glasses for glasses that have a base curve
> of 3.0.
**panto tilt and face form of both frames should be compared.
midwe...@yahoo.com
> >The optical centers on the new reading glasses are almost at the
> >centers of my eyes. So, the new reading glasses are OK as far as the
> >optical centers are concerned. However, the old reading glasses'
> >optical centers are below the centers of the eyes (by approximately 4
> >to 5 mm).
>
> The OC should be about 4mm to 5mm below the pupil, along with about 10 degrees
> of panto (lens tilt). This aligns the optical axis of the lens with the center
> of rotation of the eye. Failure to do so will induce power and astigmatic error
> on and off-axis. I would also use the distance instead of the near PD for the
> reading glasses.
>
> It sounds like the frame is sitting too high on the bridge, although the
> vertical OC can be placed lower if needed. I would prefer a frame that fits in a
> way that places the vertical center below the pupil.
>
So, are you saying that the optical centers in the new glasses are
SUPPOSED to be 4 - 5 mm below the centers of the pupils?
So, when the optician is measuring the OC height, is he supposed to
measure from the top of the frame to about 4 - 5 mm below the center
of the pupil?
Also, can you please explain panto tilt, optical axis of the lens,
center of rotation of the eye, and astigmatic error? You're throwing a
lot of jargon at me, and I'm having trouble understanding it. :)
> >The optician also said that the new lenses are smaller than the old
> >lenses and that I'm noticing the bottom of the new lenses whereas I
> >had not noticed the bottom of the old lenses. In other words, I would
> >have to get used to the new glasses.
>
> That doesn't make sense, using a narrow frame that sits high for the reading
> glasses. I'm sure the frames look very nice on you though.
>
I'm not sure that I understand. Are you saying that the frame must be
huge for the glasses to feel comfortable? My old reading glasses
(-4.25, -4) have a huge frame, and they are comfortable. However, this
frame is very huge and ugly. I was hoping to get a pair of glasses
with almost the same prescription (-4.25, -4.25), but with a small
frame (to match the modern style).
By the way, my -5.25, -5.25 distance glasses have a small frame, and
they are comfortable. And, if I recall correctly, the optical centers
of the -5.25 glasses are right at the centers of my pupils. The
centers are not 5 mm below the pupils.
>
> Lens clocks are calibrated for crown glass, and will give inaccurate readings on
> materials with different index of refraction, unless you modify the reading to
> account for this. Lens clocks will always give inaccurate results with aspheric
> surfaces.
>
An optician at the store where I bought the -4.25, -4.25 glasses
showed me that the lens clock was calibrated by putting the lens clock
onto a flat table at the store. The lens clock read "0". So, according
to the optician, the lens clock was calibrated because the clock
properly measured the base curve of a flat surface.
I do not know whether any optician who measured the base curves of my
pairs of glasses, adjusted the measurements because the glasses were
not crown glass.
>Assuming there are no other issues with the lens
> fabrication, and that the reading glasses were prescribed by an eye doctor, I
> would concentrate on the OC position and frame selection. It would probably be
> best to avoid polycarbonate to rule out any sensitivity to chromatic aberration.
>
Well, I am already avoiding polycarbonate lenses. :)
Thanks for your reply.
midwe...@yahoo.com
> panto tilt and face form of both frames should be compared.
What are panto tilt and face form?
Robert Martellaro
>On Aug 21, 11:53?am, Robert Martellaro <rob...@nospam.com> wrote:
>> >The optical centers on the new reading glasses are almost at the
>> >centers of my eyes. So, the new reading glasses are OK as far as the
>> >optical centers are concerned. However, the old reading glasses'
>> >optical centers are below the centers of the eyes (by approximately 4
>> >to 5 mm).
>>
>> The OC should be about 4mm to 5mm below the pupil, along with about 10 degrees
>> of panto (lens tilt). This aligns the optical axis of the lens with the center
>> of rotation of the eye. Failure to do so will induce power and astigmatic error
>> on and off-axis. I would also use the distance instead of the near PD for the
>> reading glasses.
>>
>> It sounds like the frame is sitting too high on the bridge, although the
>> vertical OC can be placed lower if needed. I would prefer a frame that fits in a
>> way that places the vertical center below the pupil.
>>
>
>So, are you saying that the optical centers in the new glasses are
>SUPPOSED to be 4 - 5 mm below the centers of the pupils?
Yes, assuming the panto (lens tilt- from the side, the front of the glasses
look like this / ) is 8 to 10 degrees. In most cases, the OC should be lowered
1mm for every two degrees of panto.
>So, when the optician is measuring the OC height, is he supposed to
>measure from the top of the frame to about 4 - 5 mm below the center
>of the pupil?
Pupil or OC height is measured from the bottommost part of the lens/frame. It
can also be referenced above or below the geometric center.
>Also, can you please explain panto tilt, optical axis of the lens,
>center of rotation of the eye, and astigmatic error? You're throwing a
>lot of jargon at me, and I'm having trouble understanding it. :)
See above for panto. Power errors and marginal/oblique astigmatism will cause
blurred and uncomfortable vision.
>> >The optician also said that the new lenses are smaller than the old
>> >lenses and that I'm noticing the bottom of the new lenses whereas I
>> >had not noticed the bottom of the old lenses. In other words, I would
>> >have to get used to the new glasses.
>>
>> That doesn't make sense, using a narrow frame that sits high for the reading
>> glasses. I'm sure the frames look very nice on you though.
>>
>
>I'm not sure that I understand. Are you saying that the frame must be
>huge for the glasses to feel comfortable? My old reading glasses
>(-4.25, -4) have a huge frame, and they are comfortable. However, this
>frame is very huge and ugly. I was hoping to get a pair of glasses
>with almost the same prescription (-4.25, -4.25), but with a small
>frame (to match the modern style).
Somewhere between huge and very small will be about right. How small you can go
depends on how well the frame fits (vertical positioning and distance from eye
to lens) and how much discomfort you can tolerate. For instance, you don't want
to look down to read and have the bottom of the frame in your way. This should
be discussed when the frame is selected.
>By the way, my -5.25, -5.25 distance glasses have a small frame, and
>they are comfortable. And, if I recall correctly, the optical centers
>of the -5.25 glasses are right at the centers of my pupils. The
>centers are not 5 mm below the pupils.
That'll work on the distance gaze if there is very little or no panto, and may
still be acceptable to the wearer over a fairly wide range of angles, especially
at this low/moderate power, and if the lens is not aspheric. However, if there
is no panto, then the vision on the downgaze will suffer, the floor will not
look level, and so on.
>> Lens clocks are calibrated for crown glass, and will give inaccurate readings on
>> materials with different index of refraction, unless you modify the reading to
>> account for this. Lens clocks will always give inaccurate results with aspheric
>> surfaces.
>>
>
>An optician at the store where I bought the -4.25, -4.25 glasses
>showed me that the lens clock was calibrated by putting the lens clock
>onto a flat table at the store. The lens clock read "0". So, according
>to the optician, the lens clock was calibrated because the clock
>properly measured the base curve of a flat surface.
It should read zero if there is no curve, otherwise the clock is damaged.
>I do not know whether any optician who measured the base curves of my
>pairs of glasses, adjusted the measurements because the glasses were
>not crown glass.
Just to make it more complicated, the BC listed on the package is not even the
true curve. For instance, the manufacturer might list a BC of +6.00 but the true
curve is +5.42. Regardless, the correct curve is the one that minimizes off-axis
power error and astigmatism, and that information comes from the manufacturer of
the lens blank.
With your Rx, on a spherical lens design, differences of .75D on the base curve
will result in power and astigmatic errors of about .12D, an amount that should
not be noticeable.
>>Assuming there are no other issues with the lens
>> fabrication, and that the reading glasses were prescribed by an eye doctor, I
>> would concentrate on the OC position and frame selection. It would probably be
>> best to avoid polycarbonate to rule out any sensitivity to chromatic aberration.
>>
>
>Well, I am already avoiding polycarbonate lenses. :)
>
>Thanks for your reply.
Hope this helps
midwe...@yahoo.com
> Yes, assuming the panto (lens tilt- from the side, the front of the glasses
> look like this / ) is 8 to 10 degrees. In most cases, the OC should be lowered
> 1mm for every two degrees of panto.
>
I went into a Lenscrafters store and had all three of my glasses
measured for 1) panto tilt and 2) the distance between optical center
and center of my eye.
-4.25, -4 glasses (old reading glasses) :
panto tilt: 8 degrees
optical center: 6 mm below center of eye
-4.25, -4.25 glasses (new reading glasses) :
panto tilt: 15 degrees
optical center: 2 mm below center of eye
An optician at the Lenscrafters where I bought the new reading glasses
tried to give them the same panto tilt that my old reading glasses
have. He ended up giving the new glasses twice the panto tilt of the
old glasses.
At my request, the Lenscrafters employee who was measuring the panto
tilt adjusted the panto tilt to 10 - 11 degrees. The optical center
remained 2 mm below the center of the eye. This adjustment resulted in
some alleviation of the discomfort I was feeling. However, I will have
new lenses made with the optical center being 5 mm below the center of
my eye (to match the 10 - 11 degree tilt).
-5.25, -5.25 glasses (driving glasses) :
panto tilt: 8 degrees
optical center: 5 mm below center of eye
>
> That'll work on the distance gaze if there is very little or no panto, and may
> still be acceptable to the wearer over a fairly wide range of angles, especially
> at this low/moderate power, and if the lens is not aspheric. However, if there
> is no panto, then the vision on the downgaze will suffer, the floor will not
> look level, and so on.
>
> With your Rx, on a spherical lens design, differences of .75D on the base curve
> will result in power and astigmatic errors of about .12D, an amount that should
> not be noticeable.
>
How do I know whether my lenses are sperical or aspehrical?
How can I tell the difference?
Robert Martellaro
You can ask the optician to have the lab save the lens envelopes. Reputable
optical companies will not sub lens designs without the optician's and client's
approval.
>How can I tell the difference?
You can't. The lab might be able to do so with a sag gauge.
You're getting to hung up on base curves and vertical OC position. The frame
needs to hold the lens in an optimum position, and offer adequate field of
vision for your needs. The lenses should be positioned in front of the eyes
properly, BCs should be ideal, and the Rx should be accurate from the doctor to
the final product.
Except for the doctor's Rx, a good optician should be able to determine the
cause of the discomfort in short order. It would probably be prudent to get a
second opinion at this point.