Manually alter fit values to account for real-world eyewear?

The fit values we get from our digital machines are wonderfully accurate, but they are most of the time used on plano frames. For mild Rxs no matter, but once we get into stronger Rxs as experience opticians we know the final pair is not going to look like the plano pair. Most recent case a -7.50 -2.00 OU, I'm putting him in 1.67 Auto 3 SV. I use visioffice and the values are good, but of course the demo lens is a plano 6 base, I'm going to get a 1-2 base lens back. Once I flatten out the frame a little to accommodate it the wrap is not going to be what it was in the demo. This is the biggest effect, but vertex is also impacted a bit. If the visioffice is perfectly accurate, its calculating the distance from cornea to eyewire. My actual lens is going to be at least 1.5mm center thickness, and if the bevel perfectly bisects the lens the back surface will be about 0.75mm nearer the cornea.

In this guy's case I did reduce the measured vertex by 0.75mm, and guesstimated a flattening of the measured wrap from 7.4 degrees to 3. (I will easily be able to bend the bridge slightly to get it to 3 degrees lens wrap, but bending it to 7 will look crappy and probably not perform as expected.)

1. Does anyone else worry about this, and make such compensations?
2. If we need to alter the "precision" digital values with out own judgment of the final form, especially in high Rx cases when accurate compensation is most important, how accurate are the final lenses?
3. Perhaps even fancy digital measurements and lenses are STILL only as good as your skilled optician, rather than the mall McTician operating the digital-face-measurer?

Boy are you right, Dan!

I don't trust the do-it-all measurement machines myself--I've seen too many of them taking up floor space with a cover over it because the staff lost faith in its outputs. Couldn't say if it was the machine or the staff using it...but no one schilling those things has ever offered to give me any hard efficacy studies on the tech. Glad to hear yours is working for you!

Had a high minus with a compensated PAL with a wider PD/smaller DBL---so the vertex I took with the distometer I knew would only tell half the story. With the lens thickness pushing the nosepads back, I had to have the lab calculate what the nosepad displacement would be. Added 2mm to vertex, and so adjusted the POW inputs from there prior to surfacing. (If the power is bad enough, this process could conceivably require repetition.)

Bear in mind folks, your vertex data inputs won't automatically adjust the power compensation on that fancy digital lens. Still have to do that yourself. If your lens rep tells you otherwise, they're confused.

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Originally Posted by DanLiv

The fit values we get from our digital machines are wonderfully accurate, but they are most of the time used on plano frames. For mild Rxs no matter, but once we get into stronger Rxs as experience opticians we know the final pair is not going to look like the plano pair. Most recent case a -7.50 -2.00 OU, I'm putting him in 1.67 Auto 3 SV. I use visioffice and the values are good, but of course the demo lens is a plano 6 base, I'm going to get a 1-2 base lens back. Once I flatten out the frame a little to accommodate it the wrap is not going to be what it was in the demo. This is the biggest effect, but vertex is also impacted a bit. If the visioffice is perfectly accurate, its calculating the distance from cornea to eyewire. My actual lens is going to be at least 1.5mm center thickness, and if the bevel perfectly bisects the lens the back surface will be about 0.75mm nearer the cornea.

In this guy's case I did reduce the measured vertex by 0.75mm, and guesstimated a flattening of the measured wrap from 7.4 degrees to 3. (I will easily be able to bend the bridge slightly to get it to 3 degrees lens wrap, but bending it to 7 will look crappy and probably not perform as expected.)

1. Does anyone else worry about this, and make such compensations?
2. If we need to alter the "precision" digital values with out own judgment of the final form, especially in high Rx cases when accurate compensation is most important, how accurate are the final lenses?
3. Perhaps even fancy digital measurements and lenses are STILL only as good as your skilled optician, rather than the mall McTician operating the digital-face-measurer?

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1. Yes.

2. Low values for tilt (generally less than six degrees), especially for SV lenses, does not require a high level of measurment accuracy. For example, the compensated power for the above Rx with five degrees of panto and wrap tilt is -7.37 -2.00 x 3 .5 PD BI. However, if the display frame had nine degrees of wrap tilt, the Rx to grind now becomes -7.37 -1.87 x 4 .5 PD BI, with the oblique astigmatism increasing more dramatically.

3. Yup. I don't trust the values they provide for pupil heights, and sometimes for the panto tilt. I'll use a pupilometer for the non-critical Rx/lens designs, but manually measure the IPDs using a realistically adjusted frame for higher powers and more complex designs, finding the wrap with Shamir's Panorameter, and Zeiss's plumb line device for panto.

Best regards,

Robert Martellaro

Now that I vocalized it, that's a nice point and I think I'll add my third question into my little toolbox of explanations why one needs a skilled optician. Any "optician" an operate the measuring machine (just like the pupilometer was designed less for accuracy and more for failsafing against lax measuring), but those are just raw values, not a correct final result. An optician needs to determine the true form of your final eyewear and compensated for that as well. Hadn't really thought about that concretely until now.

You'd think the tools would have an option to input the prescription and it would be able to adjust the values accordingly. Even if it used an averaged default for the adjustment, it would be better than using a plano demo lens for for the reference point.

Plus or minus 2 units tolerance for all panto and wrap values

Out of curiosity, wouldn't your lab compensate vertex distance based on Rx? Seeing as they are the ones choosing the lens blank, it would make sense that they would be responsible for altering that value if necessary. Might be a question worth asking. As far as wrap and panto, unless the lens is DRASTICALLY changing the shape of the frame, the values for wrap and panto should be pretty much the same as your inputs. However, if the lens is changing these fitting values drastically, it would probably just be a better idea to select a different frame.

At our office, we take measurements with hand tools. We do not alter these values before sending them to the lab, and this does not seem to affect our lenses in any significant way.

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Originally Posted by bretk0923

Out of curiosity, wouldn't your lab compensate vertex distance based on Rx? Seeing as they are the ones choosing the lens blank, it would make sense that they would be responsible for altering that value if necessary. Might be a question worth asking. As far as wrap and panto, unless the lens is DRASTICALLY changing the shape of the frame, the values for wrap and panto should be pretty much the same as your inputs. However, if the lens is changing these fitting values drastically, it would probably just be a better idea to select a different frame.

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If you are edging the lenses the lab can't be sure of the vertex, the bevel/frame fit would effect that. As to the 2nd point it also depends on how they are edged. If Bevel curve isn't matched exactly for one reason or another then it will alter the wrap, and should be considered - if you want it to approximate perfection anyway.

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Originally Posted by Tallboy

If you are edging the lenses

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Thanks for checking me on that, I hadn't considered in-office edging. My own paradigm at play there.

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Originally Posted by bretk0923

Out of curiosity, wouldn't your lab compensate vertex distance based on Rx? Seeing as they are the ones choosing the lens blank, it would make sense that they would be responsible for altering that value if necessary. Might be a question worth asking. As far as wrap and panto, unless the lens is DRASTICALLY changing the shape of the frame, the values for wrap and panto should be pretty much the same as your inputs. However, if the lens is changing these fitting values drastically, it would probably just be a better idea to select a different frame.

At our office, we take measurements with hand tools. We do not alter these values before sending them to the lab, and this does not seem to affect our lenses in any significant way.

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I've never known a lab who stepped into that role. It's always been up to me to tell them what power to order. They can't take responsibility for the power redaction since they can't find out vertex values of the exam anyway. (The POW defaults aren't assumptive exam vertex values, just default "vertex-as-worn"s.)

You have a practical point on steering clear of frames of that test your defaults. But for high powered Rx's, it's just a part of the job--and correcting bad fits in historical pairs is often incumbent upon us to fix. Sometimes that's in the direction of a 'default.' Sometimes it's not--especially when it takes a zyl to fix a metal fitting issue.