Do I need vertical centration?

[nicola]

My optician has determined the following prescription.

R) +0.50 -3.00 4 L) pl -3.25 175

The dispensing optician at another shop says all they need to know is the
width of the pupils.

However, in the past I have had some difficulty getting comfortable glasses
and I vaguely recall an old prescription saying something like "Must have
centration". (I may be recalling this slightly incorrectly.)

Bearing in mind my prescription, should the dispensing optician be marking
the optical centres on the blank frames in order to locate the vertical
postion of my pupils?

[Mark A]

Usually the Rx will contain PD, but any decent optician should be able to do it also. The optician has to
do the fitting height, so if they can do that, they can also do PD. If your optician expresses any
hesitation about taking the PD measurement, I would take that as a hint and go somewhere else to
have glasses dispensed.

[dumbstruck]

Do you mean vertical or not? PD is an ordinary thing which implies to me horizontal. There are even ways you can approximate it for yourself if not on any of your previous prescriptions. But just think about the physics yourself for a minute.

You have a very striking cyl correction which must put the thick axis of lense either vertical or horizontal. I would think vertical (don't have time to look this up), but if it was horizontal that might suggest you may have a whole new issue for alignment which is vertical alignment within the frame. I would think you should ignore the frame center which is arbitrary and probably even dysfunctional for style reasons, and focus on your actual line of sight center.

Look at older glasses... if the thick axis is vertical, then it may simply be a horizontal issue where you need particularly exact pd (pupil distance), I don't know the official answer to all this, but I surely do know it helps to think it thru yourself, as well as seek the party line which isn't always appropriate.

[dumbstruck]

On Thursday, May 30, 2013 9:01:30 AM UTC-10, dumbstruck wrote:
> Look at older glasses... if the thick axis is vertical, then it may simply

Forget my thick-thin directions, which may be reversed because you have minus correction. The concept is super super simple... work out the way this is expressed in old glasses yourself. You have cylinder lenses which are symmetrical in one axis... either vertical or horizontal. The alignment of your eye is unimportant along the (unchanging) length of the cyl, but very sensitive along the width. Look for symmetry... I suppose there will be 2 points that are especially thin either vertical or horizontal. The other direction will be an increasing thickness. In that case the alighment from thin to thin spot is the least important. Think thru the PRINCIPLE, not the written details which I don't have time to specify.

[Mike Tyner]

"Science_Research" <otis...@embarqmail.com> wrote
> Now that was not difficult.

Nor was it very helpful. Nicola was not asking about progressives, and his
lenses have virtually no power in the horizontal meridian so Prentice's rule
doesn't really apply, does it? When you have no education or experience in
a field, your answers add more confusion and help nobody.

Nicola, your lenses are thickest at the top and bottom. Vertically centering
your lenses will divide the thickness so the top of the lens is
approximately as thick as the bottom of your lens. If you order a pair
online, this is what they'll probably do. If you have them made by a local
optician, they will place the lenses at an equal height that best
approximates the height of your pupils in the frame. This is done because
your lenses create a little barrel distortion above and below, and it may be
easiest to get used to this when the distortions are equal above and below.

It isn't absolutely critical where they place the optical centers because
you don't always look dead level through the centers of your lenses, do you?
It will suffice that they simply get them at the same height left and right.

-MT, OD

[Mark A]

Placement of optical centers at the correct fitting height is important for aspheric lens designs. That
would include all progressives and aspheric single vision lenses. Given the moderate lens power in the
Rx of the OP, it is not likely (although possible) that an aspheric lens design would be chosen.

[nicola]

Thank you everyone. I didn't ask my question clearly! Here is my problem
restated:

My prescription is: R) +0.50 -3.00 4 L) pl -3.25 175

The dispensing optican measured the distance between the centres of my
pupils.

He said that once I had chosen my frames, one of the assistants in the
store could place the order and send them away to be made up. He also
mentioned that it would not be necessary to mark the *vertical* position of
my pupils on the frames.

Is he correct that the vertical location does not need marking? I have a
lot of astigmatism and wonder if it would give a better result if the
vertical location of my pupils were marked on the frames for the lab to
use.

[Mark A]

On Sat, 01 Jun 2013 10:52:05 +0100, nicola wrote:

> Thank you everyone. I didn't ask my question clearly! Here is my
> problem restated:
>
> My prescription is: R) +0.50 -3.00 4 L) pl -3.25 175
>
> The dispensing optican measured the distance between the centres of my
> pupils.
>
> He said that once I had chosen my frames, one of the assistants in the
> store could place the order and send them away to be made up. He also
> mentioned that it would not be necessary to mark the *vertical* position
> of my pupils on the frames.
>
> Is he correct that the vertical location does not need marking? I have
> a lot of astigmatism and wonder if it would give a better result if the
> vertical location of my pupils were marked on the frames for the lab to
> use.

Vertical height is the same as fitting height. A correct fitting height is very important for aspherical
lens designs, but not so important for spherical lenses. However, if it were way off, it could affect even
a spherical lens design. But that is not likely to be the case in most situations. But if you really wanted
it done, then they should do it for you if you ask them.

[nicola]

Thank you for the info. I will ask if they intend to use aspheric lenses
(although you said in an earlier post in this thread that they may not on
account of my astigmatism). If they are using aspherics then I will insist
they detrmine and mark the fitting height for the frames.

I hope I have got that right!

[Mark A]

On Sun, 02 Jun 2013 12:25:06 +0100, nicola wrote:

> Thank you for the info. I will ask if they intend to use aspheric
> lenses (although you said in an earlier post in this thread that they
> may not on account of my astigmatism). If they are using aspherics then
> I will insist they detrmine and mark the fitting height for the frames.
>
> I hope I have got that right!

My assumption that you would not get an aspheric lens design was not related to your astigmatism. It
was related to your sphere power, which is only moderate. Single vision aspheric lenses are sometimes
used for high power lenses to make them thinner. Progressive lenses are aspheric by definition, and
fitting height is critical for these type of lenses.

[Mark A]

On Sun, 02 Jun 2013 15:20:04 +0000, Mark A wrote:

> My assumption that you would not get an aspheric lens design was not
> related to your astigmatism. It was related to your sphere power, which
> is only moderate. Single vision aspheric lenses are sometimes used for
> high power lenses to make them thinner. Progressive lenses are aspheric
> by definition, and fitting height is critical for these type of lenses.

Actually I should have said your sphere power is quite low (not moderate) since it is only +0.50/plano.

[Mike Tyner]

"Science_Research" <otis...@embarqmail.com> wrote

> I don't know what you mean by, "centration".

Then you have no business answering questions.