# Optical Forums > Ophthalmic Optics >  Monocular decentration?

## Michael Walach

Need to get a few opinions.

OD: -16.50
OS: Ballance lens
Frame PD (A + DBL): 69
Pacient PD: 62

My feeling is that decentration is not necessary.

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## Jedi

Michael,
You would still want to place the O.C. in front of the right eye, as per usual and decenter the left for cosmetic purposes.

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## Michael Walach

Well, a person with vision in one eye only does not really have a fussion problem and does not lookat objeccts streight forward like biocular vision pacient. A way back when I used to specialize in cataract corrections, when a pacient had correction in one eye only, we positioned the lens in the center in order to get the thinest and lightest lens possible??? That is the reason I am wondering.
In minus correction, I feel, that should be the same. No problem with horizontal or vertical prism since the vision is in one eye only :Confused:

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## chip anderson

The lens should aways be positioned in such a way that the patient is looking though the center of the lens in dead forward gaze (except when prism is *required* ).  Cosmetics be damned.  If you move the center the patient will either have to use his muscles to displace his eye in order to see forward or have to move his head to search for the clearest image.  The stronger the lens is the more this is true.  A +or- 12 moved only 
2 mm  induces 2.4 prism, this is a lot of image displacement.  The patient will be reaching for forks and getting spoons.

Chip

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## Stopper

Chip is right on with this. Also think of the optical aberrations that you are inducing by not placing the optical center in front of the patients normal line of sight. -16.50 and I assume you are using polycarbonate(or similar)since the patient is monocular, I sure wouldn't want to be looking off center through that lens. Can you say chromatic abberation.

A -16.50 patient probably dosen't have any illusion that his/her lenses will be thin so not decentering a couple of milimeters isn't really going to make the lense that much more cosmeticly appealling.

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## Shutterbug

I agree with the last two posts in general because it is logical to place the OC in a normal position for patient comfort.  However the person with sight in only one eye will compensate with slight head movement the same way that progressive wearers compensate at near point with a slight tilt of the head.  This becomes so natural that it is not even noticed.  OC placement is not critical because of this (don't go wild now :p ) and if there is considerable cosmetic advantages the patient will do fine with a less than perfect PD.

shutterbug

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## ammoe2

:hammer: 

Keep In mind if this is a bifocal patient that sometimes monoccular patients will not converge as much so don't use the standard 3 split. 

Take the near pd as well as the distance

;) Low vision Ammoe

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## mrba

I disagree with shutterbug.  Optically there is no prism induced, however, as the patient looks off of the OC they will be looking through an area of more distortion.

Its a good idea to take mono pds and oc heights, to get the OC as close to being over the pupil as possible.  This will afford the patient the most crisp vision possible.

As far as cosmetics go... Ahem... a hose job is a hose job...I have had luck with the optima 1.66 bi-concave lens on these.

Lanvin (I think made by Logo if they are even still around) has a great line of small eye size big bridge frames for low minus. (ie: 40/28 eye size)

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## StephenD

The monocular centration is relatively unimportant as the patient will normally turn their head slightly to one side when looking straight ahead, thereby maximising their visual field.

Much more important with a high powered lens such as this is the back vertex distance and the lens form.   Good luck!

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## Texas Ranger

First, where do you draw the line on 'centering' the lens, for cosmetic reasons?  what if the frame pd was 74 instead of 69? how far should the chap turn his head to see through the center of the lens? I believe that the lens should be decentered 3mm in to avoid unwanted prism, that wasn't prescribed. seems like there would be enough for a monocular high myope to deal with, without having to compensate for 3-4 D. of base in prism?

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## Stopper

mrba,
I'm not sure how you can say that no prism is induced. One of the definitions of the optical center is " the point where the measured prism is zero". So if the patient is not looking through the OC then he/she has to be looking through some prism. At least thats the way I remember it. :cheers:

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## mrba

In order to have prism imbalence (which is what we are concerned about) you must have 2 eyes!  In this example one eye is a balence, for all intent and purposes blind, hence any decentration doesn't cause prism imbalence.

It is possible that even though there is no acuity in the balence eye, it my require a specific center, because that eye may have a tiny bit of perception, and hence sensitive to imbalence.  This is rare however and is usually specified in the RX, and now that I'm typing this, I'm not sure if it is specified here.

One thing I don't get from any of these posts is the concept of looking to the side or inward or whatever.  Unless the patient has a problem with their gaze, they should look straight ahead, unless they are converging to read.  The amount of decentration will effect any distortion they expirience which they should adapt to...

Ok never mind, it is possible the patient will look for the sweet spot.  I defer to my pevious post about mono pd's and oc's for the best optics... and to hell with the stupid cosmetics per my previous post as well.

But no Prism (inbalence)

:)

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## Diane

Prism IS induced.  There may not be any imbalance with monovision but , as Chip stated the image WILL be displaced and the lens should be decentered, properly.   The patient should not have to COMPENSATE for finding the best vision.  I also agree that for cosmesis, the balance lens should be decentered, unless the non-vision eye is deviated as well and re-centering that lens could have a positive result on the appearance of the eye.

Diane

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## Texas Ranger

mrba, you say we're just concerned with "prism imbalance", but that's no so. prism is prism. if it wasn't prescribed, you need to compensate the decentration to eliminate it, period, it is not a "cosmetic" issue at all. it is an RX issue. unless the dr. ordered 4D base in prism?

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## mrba

Oh dear...

The image displacement in this case is minimal.  Please refer to my previous post where I tell you how I would actually fit the lens.

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## Stopper

mrba,
I agree, no prism imbalance and i agree with your post on fitting this patient with OC in front of the pupil. 

But if the lens is not fitted as we agree, the patient will be looking through some prism(not prism imbalance) plus the other abberations you mention. 
Not trying to be argumentative, just clarifying:cheers:

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## Darryl Meister

Sheedy did a study on yoked (equal) vertical prism a few years back that demonstrated that wearers generally reject 4.0 diopters of yoked vertical prism, probably because of excessive postural adjustments and such. (Excess prism may also increase blur and distortion.) Consequently, it is possible for the wearer to reject excessive amounts of prism, even if there is no prism imbalance. If you do play around with decentration in order to improve cosmetics for a monocular patient, I wouldn't go overboard, unless the patient has already adopted a slight head turn towards the missing eye.

Best regards,
Darryl

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## jediron1

Have no idea where Mrba got his training. Chip is right, there will be a displacement problem if these lenses are not properly placed before the patients eye's. All you have to do to figure the prism in the lens is use Prentice's Rule. By using Prentice's Rule "the prismatic effect can be determined on any point on a lens using this formula". The formula is P=dXD/10. So you have distance from the oc in mm times the the power divided by 10. If you are going to be equal on both sides in our example you have 3.5mm of decentration. Taking our formula that comes out to be 5.6 prism. That is a lot of prism for any one to over come one eye or not. Also ever think about prism thinning the lens to reduce thickness? Just a thought. :drop: :bbg:

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## mrba

Hello... This isn't exactly advanced optics.

I said OC's and PD's should be measured perfectly!!!! (and to hell with cosmetics)

Prentices rule

Power(decentration off of oc)

Example 1

power=4 million diopters
decentration off of the OC= O (becasue you fit it the way I said to fit it)

Total prism= 4million(0)=0

Hello no prism.  And I have thought about prism thinning  increaseing distortion!!!!!!

And if you want my real opinion hand the guy a cane and get it over with!!!!!:finger:

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## mrba

Darryl I thought the sheedy study showed rejection of three diopters.  I don't thin my progressives more than 2.75 on the high high minus for that reason.

Prism always increases distortion.  I did a pair of 12 BO OU for a lady with graves disease and she said her image was perfect, but had classic distortion complaints.

By the way Laramy K did those lenses and did a great job.  try to get a pair of those from the evil empire's labs!!!

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## Darryl Meister

> Darryl I thought the sheedy study showed rejection of three diopters.


I don't have the article in front of me, but I'm pretty certain that most people rejected 4.0 diopters of prism, and very few people rejected 2.0. Consequently, 3.0 is probably a reasonable upper limit to maintain...

Best regards,
Darryl

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## jediron1

Hey you don't want to decenter the lens so be it. Do what you want. If this were your customer you would have a disatisfied patient and would likely have to refund. So you do what ever you want. The original said frame PD 69 and person's 62. Meaning if
split equally would have 3.5mm decentration per eye with 5.6 prism on the OD. But you do what ever you want. :Mad:  :finger:

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## Darryl Meister

> Hey you don't want to decenter the lens so be it. Do what you want.


Are you speaking to someone, in particular, Jediron? I thought that all of you had already reached a consensus to decenter the lens normally...

On a related note, few wearers actually look through a spectacle lens at a point with no prism. Even when the person is looking straight ahead (in primary gaze), he/she will most likely be looking through at least a small amount of vertical prism, unless you are taking vertical fitting height measurements for every job. Prism is hard to avoid in a prescription lens.

Best regards,
Darryl

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## mrba

In my lab, I have an account to that specifys OC on every job...

"+1 readers with an oc at 16 please"


oh yeah baby.

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## Joann Raytar

Darryl,

If this person is fit with an aspheric, would that create even more problems if the lens isn't fit on center?

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## mrba

Im not Darryl but...

If fit off of the OC a fully aspheric lens is going to be a touch off power.  In this case power in a -16 is going to be vertex sensitive and whats a .25 here or there anyways when you are a -16?

Not sure about additional distortion thoough...

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## Darryl Meister

Just remember that, unlike lenses with a spherical base curve, aspheric lenses have a "center" or pole. The asphericity increases away from this center, and ideally this center should be placed in front of the pupil (allowing for pantoscopic tilt and that sort of thing). This is why you cannot decenter to induce prescribed prism in an aspheric or grind prism for decentration, and why you must pay more attention to centration, in general, with aspherics.

The sensitivity of the design to this sort of thing will vary depending upon a number of factors, but being a couple of millimeters off probably won't matter too much for most designs.

Best regards,
Darryl

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## chip anderson

Jo:  This isn't Darrell but yes, a lot more.  

Do you wonder why people ask questions and have no intention of listening to or paying any attention to the answers even when all the replies have a common no?

Chip

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