# Thread: compensating Rx for Vertex difference

1. ## compensating Rx for Vertex difference

Curious as to how many out there compensate Rx's when there is a difference from the refracted VD to the fitted VD

2. Only in really high cases, like +/- 7 or 8.

I refract at 14 (I think) and if you fit glasses properly you'll be OK.

It's the weird combos like..."I want to wear my glasses---here!" and a -8.00 that will get my calculator going. Even so, how do you accurately measure frame vertex without a special device that no one will give me for free?

3. the patient i have is OD -12.00-1.75 OS -18.25-1.75 refracted at 12mm fitted glasses at 18mm measured with a distometer. weird i know, but it caused me to wonder how many opticians would actually compensate it.

4. most doctors won't tell you what the refracting vertex is on the rx, so you pretty much have to play a guessing game. I would say in most cases you're ok, but if you note the fitting is not "standard" and looks unusual, you're best to compensate for it. That rx that you had would be the type of Rx you'd want to. I'm with DrK... high Rx yes, but only if the fitting is really different.

5. The problem is the wearing vertex is seldom known until lenses are made. Using a distometer on sample lenses will give you poor results. Or am I missing something?

6. Measurements can be made by placing a piece of tape across the back of the frame and measuring from a closed eye lid to the tape. Then you add approximately 3mm for the thickness of the lid and add the vertex depth of the back curve which you can easily calculate. Glasses move on the patient face so trying to do this on moderate rx’s is time not well spent.
In years gone by when there were no implants and eyeglass lenses were made above +10.00 for those post cataract patients this was a common practice.

7. Lensman11, Thanks for that advice

8. Originally Posted by Lensman11
Measurements can be made by placing a piece of tape across the back of the frame and measuring from a closed eye lid to the tape. Then you add approximately 3mm for the thickness of the lid and add the vertex depth of the back curve which you can easily calculate. Glasses move on the patient face so trying to do this on moderate rx’s is time not well spent.
In years gone by when there were no implants and eyeglass lenses were made above +10.00 for those post cataract patients this was a common practice.
I was fitting cataract pts back in the 80's at an eye hospital where we were fitting 15 -18 pair per day and we would be given the RVD.

9. Originally Posted by Lensman11
Measurements can be made by placing a piece of tape across the back of the frame and measuring from a closed eye lid to the tape. Then you add approximately 3mm for the thickness of the lid and add the vertex depth of the back curve which you can easily calculate. Glasses move on the patient face so trying to do this on moderate rx’s is time not well spent.
In years gone by when there were no implants and eyeglass lenses were made above +10.00 for those post cataract patients this was a common practice.
I think the allowance for lid thickness is typically 1mm.

B

10. +1

11. Originally Posted by Randle Tibbs, ABOM
the patient i have is OD -12.00-1.75 OS -18.25-1.75 refracted at 12mm fitted glasses at 18mm measured with a distometer. weird i know, but it caused me to wonder how many opticians would actually compensate it.
Hard to understand how the refractor head can sit that much closer than the frame. Might be a good idea to refract over the old Rx to see what's what.

Best regards,

Robert Martellaro

12. Originally Posted by Randle Tibbs, ABOM
the patient i have is OD -12.00-1.75 OS -18.25-1.75 refracted at 12mm fitted glasses at 18mm measured with a distometer. weird i know, but it caused me to wonder how many opticians would actually compensate it.
The wheel calculator that comes with a distometer would show a 6mm compensated Rx increase for OD of about -1.00 at -12.00 and over -2.00 increase in OS for -18.00, so yes... a qualified optician must compensate for a 6mm VD change in this high Rx.
And this patient didn't wake up with a -12.00 and -18.00 RX, so consider using his old glasses for a vertex distance measurement and add 1mm for thickness of eyelid. Every optical should have a distometer.

13. Originally Posted by tmorse
The wheel calculator that comes with a distometer would show a 6mm compensated Rx increase for OD of about -1.00 at -12.00 and over -2.00 increase in OS for -18.00, so yes... a qualified optician must compensate for a 6mm VD change in this high Rx.
And this patient didn't wake up with a -12.00 and -18.00 RX, so consider using his old glasses for a vertex distance measurement and add 1mm for thickness of eyelid. Every optical should have a distometer.
Agreed. I don't know how anyone could have a chance of making successful glasses for this Rx without doing this.

14. i agree with all that has been said. Mainly, I was just curious to how many opticians still remember how to and why to calculate the effective power and when to do so. Also, you'd be surprised how many do not know what a distometer is or how to use the compensating wheel or that you also need to calculate for the cyl as well.
really wasn't expecting this much response, but glad to see it.
thank you all.

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