Optical center placement

[Himyope]

A topic of discussion at the office lately: In a SV job, under what circumstances would you specify an OC height instead of placing it at geometric center. I have my opinion, the OD I work for has his opinion and of course so does everyone else.
Naturally, I have the greatest respect for the members of Optiboard and would like to know what you all have to say! I figured it was time I got in on the action, I've been reading the board for months but was a little reluctant to jump in right away, things often get a little crazy here, I'm not sure I'm ready for it.

[chip anderson]

Not a bad idea anytime. Especially important when high correction, especially high cylinder present in Rx. Some people will actually see rainbows around objects if OC is not placed on or near the vertical visual axis.

Watch out for large frames where the eye is far away from the geometric center.

Chip(aberrated) Anderson

[John R]

Hi there and welcome to this forum :D
Well as a lab rat the more info we get the better we can make the lenses, but i sometimes wonder this to. Take my glasses for example re(4.50 +1.50 @ 165)le(+0.50 +0.25 @ 35) i think, wearing 1.66 pentax aspherics i have 4 mm of horizontal dec worked but vertical they are on geo center but if you look through the frames pupils are about 8 mm above the oc. I have no problems with them what diffrence would i see if they were glazed with the vertical oc at 8 above apart from having thicker lenses.

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Every day a grind
Every week a bind
www.iooi.co.uk

[Jeff Trail]

Chip is correct on placement in those high myopic RX's and high cylinder.. But you also need to add a few to the list....

1)aspheric's and atoric curves, you must take mono PD's and fit the OC, that is if you want to get the most benefit from the design.
2) Prism, you should fit OC's in cases where you are grinding prism, especially when you have vertical prism.
3) Even in spherical designs where the fitting OC is farther then 5 or 6 mm from the datum line.
4) not done very often, but when slabbing a SV you need to supply it as well.

In the majority of spherical lens, especially with the trend of smaller frames, then fitting an OC is not usually necessary mainly because the eye strikes close to the datum line or with in tolerance where it has no effect. But if you do prefer it then you can just as easily as not.
The first has to do with the nature of the curve and how it effects the optics of the lens. Should be fitted OC's as well as mono PD, especially since the geometric center and the mechanical center have to be in the same place (can NOT use ground decentration in these type curves)
The second comes in handy when the eye is striking away from the datum as well.. mainly because it will have an effect on the eye's perception of "prism". If you wanted to get really technical, you could use Prentice's rule to figure out how much prism you are adding or deleting by having improper OC location.
The third is not as big an issue as it used to be since the trend is towards smaller frame sizes.. but with a "quick" visual you can get a fairly good idea if the OC is going to be located further then 6 or so mm from the datum (where we will put the OC 99.5% of the time, unless requested OC's are supplied. I have been asked a number of times over the years "why is the OC on the datum?" .. it just increases the amount of effective cutout diameter of the lens and cosmetics as why we do it. (especially when using FSV blanks)
The fourth is pretty well self explanatory I would think :)
BTW a good rule of thumb for optical center placement in aspherical and atorics is 1 mm of drop for ever 2 degree's of panto tilt. Hope some of this might add fuel to your debate with your OD :)

Jeff "always thought it was just as easy to do it right the first time as doing it wrong" Trail

[bblack]

If you have to raise optical centers you should pick out a different frame. We have so many frames to pick from in todays market. Excellent optics is vital but my customer also wants great esthetics. Yes, raising OC's it is the right thing to do but you should not need to.

Bill

[EyeManFla]

Assuming there is a reason for not having the eyes centered in a high Rx, to only other time I would change the placement of the O.C.s on single vision is for custom made shooting glasses and glasses for pool shooters.

[Laurie]

Hi,

Wholesale lab people are telling me that they have seen a big jump in re-do's due to sv optical height placement. Most people, when looking above the datum line, are seeing through the oc because of the 2 degree tilt/1mm up oc placement thing.

When to do it:

anisometropes or antimetropes
any high-tech design ( aspheric, atoric, high index).

Here's a great tip to compensate for tilt/oc height (Martin's rule of tilt) WITHOUT having to go through the trouble of trying to measure panto, etc...

(This tip is in the new edition of the "blue book", System for Ophthalmic Dispensing)...

Adjust the glasses with proper amount of panto, face form, and wrap. Then, with the glasses on, ask the client to lift their head until you see the eyewire/lenses as STRAIGHT UP AND DOWN, orthoscopic. Then, when you see the eyewire as orthoscopic (90 degrees from floor and ceiling), then spot the pupils. Because you made the markings with the eyewire "plane to the face", it will automatically compensate for the 2 degree 1mm rule.

How much fun is that.

Laurie

[John R]

Originally posted by bblack:
If you have to raise optical centers you should pick out a different frame. We have so many frames to pick from in todays market.

This maybe true but how many shops carry a very large range.
We have a small shop on the side of our lab but it only has about 200 frames not much choice there and as we get then for free you dont want to go and buy a pair from else where...

[Rich R]

I see alot of opticians putting an o.c. ht., but not compensating for the panto, result is a very high o.c. When the rx is -8.00 or so with o.c. ht of 33 in a frame with a 48 B dimension, it makes a hideous looking rx. I usually won't go more than 5mm up on an o.c. on sv lenses.
Rich R

[Texas Ranger]

There should be some common sense used here. take a look at how much base up or base down prism is involved. moving a vertical oc too much up can cause the pt to look through too much base down prism when walking,etc. and it looks really bad. seems like one of ralph drew's writhings years ago said that vertical ocs should be about 4 mm below pupil level, on average, always has worked out well doing that. oblique cyls cause some problems as do aspherics. but 3 or 4 mm below pupil usually is about right. Al.

[Pete Hanlin]

I just wanted to say "ditto" to Laurie's post. All too often, I see OCs incorrectly ordered above the datum line. In most cases, the pantoscopic tilt of the frame has not been accounted for.

Given about 10 degrees of pantoscopic tilt, it is somewhat unusual to find it necessary to raise the OC measurement above the datum line. Jeff makes a good point concerning aspheric/atoric lenses (which have a centrally located "button" that should be positioned near the visual axis). However, even then I would consider it almost as important to provide a perpendicular axis through the lens' surfaces (which is what the correct panto/decentration ratio attempts to accomplish).

As Laurie mentioned, the "easiest" method of finding the correct OC placement is outlined on page 67 of "System for Ophthalmic Dispensing (second edition)," by Brooks and Borish. Just be sure you are comfortable looking into people's nostrils! :) I would advocate that anyone wishing to gain a good fundamental knowledge of dispensing should procure a copy of this book. Read from cover to cover- and with the additional input of a trained mentor Optician or education- this volume should provide a moderately complete education/introduction to the field.

Pete