Curious as to how many out there compensate Rx's when there is a difference from the refracted VD to the fitted VD
Curious as to how many out there compensate Rx's when there is a difference from the refracted VD to the fitted VD
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?
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.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
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.
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.
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.
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?
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.
+1
Lensman11, Thanks for that advice
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