What is the max limit on induced horiz prism when edging?

[ilanh]

For those of you who edge, I would like to know what is the maximum amount of prism you are willing to induce in the horizontal meridian by using a stock blank which is smaller than the minimum blank size needed? For example, if the Rx for a pair of readers is +2.00 OU, a 5mm decentration in each eye would induce 1.0D of horizontal prism in each eye. Would this be tolerable for most people? Do you factor in whether you are inducing base in or base out prism? Whether it's for distance or reading glasses? This question addresses induced prism in spherical prescriptions and does not consider edge thickness as a factor.

[glazinggeek]

prism power tollerences in horizontal
in the power +2.00 the following applies
>0.00and=/<2.00 +(0.25+0.1 x smax)
smax is the focal power in this case u would be allowed 0.7 prism

[DragonLensmanWV]

My general rule is if I can get the required prism with 5mm or less extra decentration, then I use a stock lens.
Of course you can't do that with aspheric lenses.

[OIC]

Would this be tolerable for most people? Do you factor in whether you are inducing base in or base out prism? Whether it's for distance or reading glasses?

Generally plus Rx wearers are more tolerant of base in prism, where minus Rx wearers are more tolerant of base out prism.

[obxeyeguy]

My general rule is if I can get the required prism with 5mm or less extra decentration, then I use a stock lens.
Of course you can't do that with aspheric lenses.

I take the question to be how much "off" on the PD is acceptable when using stock lenses that are not large enough. Ilan??

[Robert Martellaro]

For example, if the Rx for a pair of readers is +2.00 OU, a 5mm decentration in each eye would induce 1.0D of horizontal prism in each eye. Would this be tolerable for most people?

Horizontal Prism Imbalance ANSI Z80.1-2005

≥ 0.00 D to ≤ ±2.75 D
±0.67 Δ Total

> ±2.75 D
±2.5 mm Total

2Δ BO might be tolerated by some, but would not be desirable, especially for prolonged use. Increasing the deviation results in decreased tolerance, and eventually zero tolerance. It's a slippery slope, and I would strongly recommend not going there.

[flexonman]

Max limit < .25 diopters in my lab. Go for zero. If it's not there don't put it there. If it has been, by error, induced before watch out people get used to things they should not have too.

ANSI is posted prior. I find that tolerance pretty substantial.

Doing things right the first time makes things far easier than having to explain an error or having to redo the job completely again =/ Your patients will appreciate your work and trust you will do right by them.

As for "what people will tolerate" that's on an individual basis. Everyone is different.

[braheem24]

I'm reading the question differently: I'm assuming an OMD already knows his patient's tolerance, So I'm assuming that's not the question.

You can induce as much prism as the blank allows, in, out, up or down. Total amount varies on frame size, patient PD and lens blank.

Sufrace or finish theoretically makes no difference.

The only consideration is for the induced chromatic abberation caused by lower abbe value materials since you'll be moving the OC away from the pupil but that holds true for surface as well as finish.

Thickness is only taken into consideration for plus lenses. Minus lenses cannot be made any thinner by surfacing prism.

[AdmiralKnight]

I second Braheem's post.. I'm assuming ilanh was asking how far do we feel comfortable decentering a lens to achieve the PRESCRIBED prism, not about prism tolerances.

Honestly, I've never thought of it. If I can get the prism by decentering, I would try it. That being said, if I saw anything more than 2.00 D I automatically think surfaced. Just for cosmetics.

[FVCCHRIS]

As previously noted, you can decenter non-aspheric finished stock lenses all you want in any direction to achieve an accurate Rx either with or without prescribed prism. The main concern is whether or not the finished blank size is sufficient to be used and then, equally important, will this give you a finished lens which is cosmetically appealing? Many times, especially in the case of plus lenses, you will have to surface those lenses in order to end up with an acceptable result. I find base curves on most plus lenses above +2.00 to be too high for a nice result. As far as unintended prism ANSI standard should not be exceeded- if the stock lens won't cut out- surface it. :)

[ilanh]

I take the question to be how much "off" on the PD is acceptable when using stock lenses that are not large enough. Ilan??

Thanks,
That is a simpler way of putting my question. The issue popped up because on several occasions we induced prism above and beyond the 0.66D ANSI standard on horizontal spherical prescriptions. I carefully followed the patients and urged them to return the glasses if they felt any problem whatsoever. I believe that the ANSI tolerance standard for horiz prism is unrealistically low. My personal threshold would be about 2D of total induced horiz prism (especially if it's base in). I have also looked at patients who have around 2D of erroneously induced horiz prism and checked to see how they do with trial frames with the same Rx and no prism. I have not found that it has changed them in any way. Vertical prism is, of course, a totally different story and I keep that as close to zero as possible.

[drk]

It's just a matter of prism, Ilan.

The p.d. can be off a gazillion mm in low power, or super-important to be precise on high power.

Post the specifics, and you can calculate the type and amount of prism.

I assume you want to outset because you don't have a big enough lens. If they're a hyperope, they're going to get BO, and that's never good, ever--instant headache. If they're a myope, a little BI won't kill, as long as it's 1^.

What if they're "near only"? Usually plus power lenses, decentering outward will cause excessive convergence. Boom!

[sharpstick777]

Is the question RE how much prism you can induce in a stock lens vs. going with a grinder?

[Fezz]

In a world of compromise...............some don't!

[ilanh]

It's just a matter of prism, Ilan.

The p.d. can be off a gazillion mm in low power, or super-important to be precise on high power.

Post the specifics, and you can calculate the type and amount of prism.

I assume you want to outset because you don't have a big enough lens. If they're a hyperope, they're going to get BO, and that's never good, ever--instant headache. If they're a myope, a little BI won't kill, as long as it's 1^.

What if they're "near only"? Usually plus power lenses, decentering outward will cause excessive convergence. Boom!

Here's how this issue came about. Before we purchased an edger we used to use a lab that gave us the option of using stock lenses vs. surfacing a job. They claimed that whenever they could use a stock lens, they would (less costly than surfacing). However, we noticed that in a huge number of cases they would claim that they could not use the stock lens for the job ("blank too small"). Eventually, we bought our own edger and our own stock lenses (from that same lab) and began to realize that in most of these cases it was a matter of a few millimeters of fudging in a low powered spherical lens. We have significantly decreased the number of jobs that "needed to be surfaced" and I literally have yet to see a complaint from the usually less than 1.5D of horizontal prism that we induce. My standard is now twice that of ANSI's

[CME4SPECS]

Here's how this issue came about. Before we purchased an edger we used to use a lab that gave us the option of using stock lenses vs. surfacing a job. They claimed that whenever they could use a stock lens, they would (less costly than surfacing). However, we noticed that in a huge number of cases they would claim that they could not use the stock lens for the job ("blank too small"). Eventually, we bought our own edger and our own stock lenses (from that same lab) and began to realize that in most of these cases it was a matter of a few millimeters of fudging in a low powered spherical lens. We have significantly decreased the number of jobs that "needed to be surfaced" and I literally have yet to see a complaint from the usually less than 1.5D of horizontal prism that we induce. My standard is now twice that of ANSI's

So you are compromising to save a buck! That's not right! If you are going to sell glasses...make them right! You have an obligation to your patient to make them correctly. Let the wally worlds, internet and the discount houses do the schlock work! Charge them for "surfaced" lenses...just do it right!

[FVCCHRIS]

So you are compromising to save a buck! That's not right! If you are going to sell glasses...make them right! You have an obligation to your patient to make them correctly. Let the wally worlds, internet and the discount houses do the schlock work! Charge them for "surfaced" lenses...just do it right!

No reason to ever deviate from those tolerances listed in the ANSI standard! Hell, that's why they call them STANDARDS! If you can make a job look good and cut out with a stock lens and be within the ANSI standards then by all means do so. Start "fudging" on your eyewears quality by allowing anything in excess of those standards and you are setting yourself apart from those of us who choose not to. :finger: Never mind what you think your patients can tolerate in horizontal prism. It's not ethical in my mind.

[DragonLensmanWV]

Why spend all that time performing a highly accurate refraction only to fudge in the execution of the lenses?
As has been said, if you can do it with a stock lens, do so, if you run out of blank (and you DO run out of blank very very quickly at lower powers) ,then get it surfaced to your standards which should be at least as stringent as ANSI's.

[Robert Martellaro]

I believe that the ANSI tolerance standard for horiz prism is unrealistically low. My personal threshold would be about 2D of total induced horiz prism (especially if it's base in).

Here's data that seems to contradict your findings.

http://journals.lww.com/optvissci/Fu...dymade.13.aspx

You're example was +2.00 DS readers. If this was a distance Rx, there would be additional base out prism induced when used for reading. Assuming 2mm convergence per eye, the total induced prism would be .8^ BO. Adding that to your rather loose standards, the total unwanted prism becomes 2.8^. Although the wearer is accustomed to the induced prism, the additional 2^ will drive most over the cliff. Boom, as drk says.

And don't be so sure that they'll come back to complain- most will go back to their old glasses, or go elsewhere. Moreover, if they come to me, after verifying the written Rx, I'll tell them that their glasses were improperly made, and that the error could have been detected during the inspection before they were dispensed. Ouch.

FYI, I feel a pull and subsequent dull ache with as little as 1^ in or out. 3^ is unbearable.

Unless you're checking everyone for binocular vision problems (no?) you'd be wise to follow the ANSI standards.

[ilanh]

No reason to ever deviate from those tolerances listed in the ANSI standard! Hell, that's why they call them STANDARDS! If you can make a job look good and cut out with a stock lens and be within the ANSI standards then by all means do so. Start "fudging" on your eyewears quality by allowing anything in excess of those standards and you are setting yourself apart from those of us who choose not to. :finger: Never mind what you think your patients can tolerate in horizontal prism. It's not ethical in my mind.

Wrong . I don't give a fig for standards that are not based on any objective clinical studies (and I've looked). Actually, I'm not sure what they based them on because there are very few actual randomized and statistically significant studies out there. I've seen this quoted Australian study before; it uses all of 8 patients (statistically irrelevant), but is a well organized and logical study. Ideally speaking it would have been better to do such a study with a far greater number of participants. Yet, even this study concludes tolerances almost triple to that of ANSI:

"The majority of participants in this study had clear and recognizable responses to prism beyond their range of tolerance. As a result, most (5 to 7) were not prepared to wear the higher amounts of prism for the required 8 h. Participants' comfort ratings confirmed that for spectacles to be visually comfortable to the majority (tolerated by 7 or 8 participants for 8 h), induced prism should be <1 Δ BU, 2 Δ BO, and 2 Δ BI."

Regardless, in my personal practice I deal with a lot of neurological patients, diplopia, cranial nerve palsies and strabismus. Over the years I've developed a feel for what levels of induced prism are well tolerated in the horizontal meridian. I've always felt that the oft cited ANSI standards are plain wrong. I confess that my own estimate of 2D prism horizontally is somewhat arbitrary, but it is based on my clinical observations. I am more than ready to modify this based on sharing cumulative experience with my colleagues.

P.S: No reason to bring in issues of ethics and holier than thou claims. This forum is a wonderful opportunity to collaborate and learn from others. Too much knee jerk reactions going on around here.

[YrahG]

Wrong . I don't give a fig for standards that are not based on any objective clinical studies (and I've looked). Actually, I'm not sure what they based them on because there are very few actual randomized and statistically significant studies out there. I've seen this quoted Australian study before; it uses all of 8 patients (statistically irrelevant), but is a well organized and logical study. Ideally speaking it would have been better to do such a study with a far greater number of participants. Yet, even this study concludes tolerances almost triple to that of ANSI:

"The majority of participants in this study had clear and recognizable responses to prism beyond their range of tolerance. As a result, most (5 to 7) were not prepared to wear the higher amounts of prism for the required 8 h. Participants' comfort ratings confirmed that for spectacles to be visually comfortable to the majority (tolerated by 7 or 8 participants for 8 h), induced prism should be <1 Δ BU, 2 Δ BO, and 2 Δ BI."

Regardless, in my personal practice I deal with a lot of neurological patients, diplopia, cranial nerve palsies and strabismus. Over the years I've developed a feel for what levels of induced prism are well tolerated in the horizontal meridian. I've always felt that the oft cited ANSI standards are plain wrong. I confess that my own estimate of 2D prism horizontally is somewhat arbitrary, but it is based on my clinical observations. I am more than ready to modify this based on sharing cumulative experience with my colleagues.

P.S: No reason to bring in issues of ethics and holier than thou claims. This forum is a wonderful opportunity to collaborate and learn from others. Too much knee jerk reactions going on around here.

It's just bad business, forget ethics, forget your clinical findings. You are tripleing the tolerance to save a few bucks your motives are wrong. If you really want to make a few more bucks try advertising, raising prices, etc.

[FVCCHRIS]

For those of you who edge, I would like to know what is the maximum amount of prism you are willing to induce in the horizontal meridian by using a stock blank which is smaller than the minimum blank size needed? For example, if the Rx for a pair of readers is +2.00 OU, a 5mm decentration in each eye would induce 1.0D of horizontal prism in each eye. Would this be tolerable for most people? Do you factor in whether you are inducing base in or base out prism? Whether it's for distance or reading glasses? This question addresses induced prism in spherical prescriptions and does not consider edge thickness as a factor.

Wrong . I don't give a fig for standards that are not based on any objective clinical studies (and I've looked). Actually, I'm not sure what they based them on because there are very few actual randomized and statistically significant studies out there. I've seen this quoted Australian study before; it uses all of 8 patients (statistically irrelevant), but is a well organized and logical study. Ideally speaking it would have been better to do such a study with a far greater number of participants. Yet, even this study concludes tolerances almost triple to that of ANSI:

"The majority of participants in this study had clear and recognizable responses to prism beyond their range of tolerance. As a result, most (5 to 7) were not prepared to wear the higher amounts of prism for the required 8 h. Participants' comfort ratings confirmed that for spectacles to be visually comfortable to the majority (tolerated by 7 or 8 participants for 8 h), induced prism should be <1 Δ BU, 2 Δ BO, and 2 Δ BI."

Regardless, in my personal practice I deal with a lot of neurological patients, diplopia, cranial nerve palsies and strabismus. Over the years I've developed a feel for what levels of induced prism are well tolerated in the horizontal meridian. I've always felt that the oft cited ANSI standards are plain wrong. I confess that my own estimate of 2D prism horizontally is somewhat arbitrary, but it is based on my clinical observations. I am more than ready to modify this based on sharing cumulative experience with my colleagues.

P.S: No reason to bring in issues of ethics and holier than thou claims. This forum is a wonderful opportunity to collaborate and learn from others. Too much knee jerk reactions going on around here.

I stand rebuked and contrite. In answer to your original post about how much unintended prism do I tolerate by using a stock lens which is too small, my answer is 0.00 diopters. If the Rx does not request prism I do not provide prism, on purpose or by "accident".

[braheem24]

Dispensed a lady today with first prism Rx.

OD: 2BI 0.5BU
OS: 2BI 0.5BD

She didnt appreciate the difference and kept going back to her old glasses at the dispensing table, even though she came in complaining of intermittent double vision.

Tolerance at the dispensing table or office may not be tolerance in life. As a prescriber ANSI does not apply.

For true research I would make myself the first guinea pig, after all I would also be the most knowledgable and provide the best feedback.

[Robert Martellaro]

Ilan,

I believe Darryl Meister had a hand in determining the ANSI standards. Maybe he'll visit this thread and share the methodology used when the tolerances for unwanted prism were established. My guess is that the .67Δ standard assured that those with healthy eyes would be free of symptoms, and that it was technically feasible to do so using equipment and expertise that was readily available at the time the standards were created.

However, the standard does allow for higher amounts of prism with higher dioptric values. For example, when the total power at 180 is 10.00 D, the 2.5mm total error standard induces 2.5Δ. So it appears that there's one standard for low powers, and other for higher powers. I suspect that this discrepancy in standards is not due to the belief that this degree of prism is easily tolerated, but is due to limits in technology that would otherwise make the lenses cost prohibitive to the end user.

Due to advances in equipment over the past ten years, I feel that instead of loosening the tolerances for the lower powers, they should stay the same, with the tolerances for higher powers tightened slightly. I also wish that all dispensaries were populated with ABOC opticians, with at least one lead ABOC-AC optician, that online eyeglass merchants get shut down, and that I had an Audi R8 and two hours of track time at Elkhart Lake on the weekends.

[MIOPE]

To me not prescribed prism i a unwanted prism. ANSI standards are the least amount of unwanted prism that is tolerated by most people. Each person is different and you may find people that tolerate more unwanted prism that what is said on ANSi standards but that does not means that everyone will tolearate that amount. Prism is part of the prescription. If no prism is prescribed why should i make the glasses with unwanted prism?. It is the same as say that because i do not have the exact prescription on stock lenses i will use a closer one.