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Thread: Effective power of lens vs compensated Rx

  1. #1
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    Effective power of lens vs compensated Rx

    Hello folks!

    I'm currently studying for the Advanced Test and I want to make sure I have a clear understanding of two different concepts (correct me if I'm wrong please!):

    Effective power is what the patient experiences as the power of the lens while wearing them. Adding vertex distance increase plus power and removing vertex distance decreases plus power (or increases minus power).

    Compensated Rx then is knowing that the effective power will be different than what is actually prescribed, and is made so that if a patient is wearing a pair of glasses at a specified vertex distance, provides the same refractive power as to what is prescribed.

    So if a patient was prescribed a -10.00 lens OD with a refractive vertex distance of 10mm but selected a frame with a vertex distance of 15mm then:

    The effective power would be less than -10.00 because there is an increase in vertex distance (more plus power).

    And the compensated Rx would be more than -10.00 to compensate for the increase in plus power.

    Does this sound about right?

    Thanks in advance!

  2. #2
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    Worth noting that there are other variables - wrap or bow angle and pantoscopic angle.

    SOME advanced lenses also compensate based on previous Rx to minimise the adaptation period. But this is rare.

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    This is not an Rx that can be done digitally or would their be any advantage to doing with a digital process. It surely could be done by any lab as as they have the fixtures to set up the prism. Most older full service labs would be able to produce this Rx. Epic lab specializes in orders of this nature contact them for a solution to your problem.

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    Quote Originally Posted by azhanli22 View Post
    Hello folks!

    I'm currently studying for the Advanced Test and I want to make sure I have a clear understanding of two different concepts (correct me if I'm wrong please!):

    Effective power is what the patient experiences as the power of the lens while wearing them. Adding vertex distance increase plus power and removing vertex distance decreases plus power (or increases minus power).

    Compensated Rx then is knowing that the effective power will be different than what is actually prescribed, and is made so that if a patient is wearing a pair of glasses at a specified vertex distance, provides the same refractive power as to what is prescribed.

    So if a patient was prescribed a -10.00 lens OD with a refractive vertex distance of 10mm but selected a frame with a vertex distance of 15mm then:

    The effective power would be less than -10.00 because there is an increase in vertex distance (more plus power).

    And the compensated Rx would be more than -10.00 to compensate for the increase in plus power.

    Does this sound about right?

    Thanks in advance!

    All correct. Good Luck!

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    Quote Originally Posted by Lensman11 View Post
    This is not an Rx that can be done digitally or would their be any advantage to doing with a digital process. It surely could be done by any lab as as they have the fixtures to set up the prism. Most older full service labs would be able to produce this Rx. Epic lab specializes in orders of this nature contact them for a solution to your problem.
    That is incorrect. There would be advantages to digital, compensated production.

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    Sorry I posted this answer on the wrong post

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    Quote Originally Posted by Robert_S View Post
    Worth noting that there are other variables - wrap or bow angle and pantoscopic angle.

    SOME advanced lenses also compensate based on previous Rx to minimise the adaptation period. But this is rare.

    Hi Robert,

    Oh yes! Sorry for the confusion. I totally forgot that "compensated Rx" have a collection of variables at play. I'm digging through the Optical Formula Tutorials book and their Compensated Rx refers to the change in prescription due to the vertex distance. I'm just hoping that the test doesn't ask us to calculate a "compensated rx" that includes all the variables. That'd be very time consuming if not down right impossible.

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