Thanks
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.
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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So....after all, what is the answer?? No one can tell?
My understanding is p=Dd,so question 1 gonna be -5x1 =-5D ,question 2 , -1.25x1=-1.25D, none of them are accepted?
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.
I agree with tallboy.
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