Thanks
need help !
Thanks
Last edited by raben4; 07-24-2017 at 06:49 AM.
It will decenter the lenses 1mm in each eye, there is a bridge difference of 2 mm, so each eye is decentered 1 mm.
Run it through Prentice's rule (which I bet is in your book somewhere) and see if the resulting induced unwanted prism is more or less than 0.67, if it is more it is not in tolerance and not acceptable, if it is less then it is acceptable.
The powers must be written in the 180 degree meridian, because that is the nature of decentration being calculated. Transpose the RX by adding the cyl to the sphere in any part of the RX that is written at 90degrees. so -4.00 -1.00 x 90 becomes -5.00 +1.00 x 180. In this case -5.00 is the power on the 180 meridian and is the power in concern for the horizontal decentration, -5.00 is the power in diopters being plugged into the Prentice's rule equation.
If the RX is already written in regards to the 180 degree meridian, you do not need to transpose before doing the calcution for induced prism. I hope that helps
Prentice's rule is a moot point for the OD. RXs over 2.75 is Plus/minus 2.5mm PRP horizontally, or 1.25mm per eye. The OS, being under 2.75, prism via Prentice will apply.
Lol yes in real world fabrication but that won't fly on their math homework. The teach wants an answer.Originally Posted by lensmanmd
Yup, real world QA...if the PD is within 1mm per eye, it will always fly, regardless of RX. No math required.
First find the power at 180. Then find out how much the lens is being displaced. Then run your Prentice formula.
1.) OD -5.00 @180; OS -5.00 @ 180; PD displaced 2mm in. (-5.00(2))/10= 1 diopter BO
2.) OD -1.25 @180; OS -1.25 @ 180; PD displaced 2mm in. (-1.25(2))/10= 0.25 diopters BO
If the OC is displaced vertically you would find the power at 90.
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Dude, it's a 2mm DBL difference, so 1mm per eye. Prism needs to be cut in half. Total prism would be .625 BO
So....after all, what is the answer?? No one can tell?
Oh I thought you wanted me to explain the how and why not actually give you the answer. Have you tried working it out using the method and formulas I mentioned? What is confusing you maybe I can clarify
My understanding is p=Dd,so question 1 gonna be -5x1 =-5D ,question 2 , -1.25x1=-1.25D, none of them are accepted?
When calculating Prentice's rule values from measurements in mm, the formula needs to account for the units of measurement:
=
Oh , so ....then the answer gonna be -0.5 and -0.125, both of them less than 0.67, so both of them will be accepted. Correct?
Yes but since both eyes are minus and both are decentered in the same direction (Inwards towards the bridge), it leads to compounding prism. When calculating the prism for wearable tolerance you have to calculate the prism induced in both lenses and add them, because prism is experienced by the whole visual system. The prism induced by the glasses is actually 1.00 prism diopters and 0.25 prism diopters. In this case the first set of lenses fails and the second set passes.
edit: The prism direction for both pairs is base out. If they had been plus lenses it would induce base in prism.
Last edited by Tallboy; 07-18-2017 at 08:54 AM.
I disagree. Prism OD is .5D, OS is .125. Combined, it is .625. Technically, it passes, as the combined prism is less than .67. However, the OD would fail on its own if just the prism were to be taken into account. .335 per eye would be the acceptable prism.
Complicating matters is the the power of the OD. It is over 2.75, so Prentice's rule can be discarded, and instead ANSI says 1.25mm deviation is acceptable, in which case, this will pass.
OP is from Sydney and and referenced ISO 21987:2011 so ANSI is out the window in this case.
LMD I think you are reading the question wrong, it is two different pairs of glasses.
1. RE: -4.00 /-1.00x 90 LE: -4.00 /-1.00 x90
2. RE: -1.25 /-1.00 x180 LE: -1.25 /-1.00 x 180
The first pair the OD and OS (RE, LE) are both over -4.00 sphere
the second pair the spheres are -1.25.
Its all that MR resin fumes you've been breathing in.
TY
I'm obtuse and reading it wrong.
Last edited by lensmanmd; 07-18-2017 at 11:41 PM.
Ha. You are correct. If not the resins, the scotch.LMD I think you are reading the question wrong, it is two different pairs of glasses.
1. RE: -4.00 /-1.00x 90 LE: -4.00 /-1.00 x90
2. RE: -1.25 /-1.00 x180 LE: -1.25 /-1.00 x 180
The first pair the OD and OS (RE, LE) are both over -4.00 sphere
the second pair the spheres are -1.25.
Its all that MR resin fumes you've been breathing in.
I'm reading this wrong. Pair 1 is off. Pair 2 is prism based and passes ISO 21987.
Citing the reasons should be easy. Just quote the standards as a reference.
Last edited by lensmanmd; 07-18-2017 at 11:40 PM.
I agree with tallboy.
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