Prentice's Rule

[Jersey]

Can someone anyone help me with the Prenice's Rule I'm lost Jersey

[optixbil]

1 diopter of power moved 10mm equals 1 diopter of prism. What exactly are you having a problem with?

[pbsE46]

Power x decentration / 10 = Prism

In the above example 1 x 10 = 10 divided by 10 = 1

Another example .25 x 10 = 25 divided by 10 = .25

Another example 1.50 x 3 = 4.5 divided by 10 = .45
which in easy terms mean in you take a 1.5d lens and move the OC 3 m/m you will end up with .45d prism.

It's really not that hard if you think about what you're accomplishing.

[ikon44]

P = c x f

P= prism
c= centration
f= power of the lens.

e.g -6.00 rx PD = 60 but lenses are set at 66mm

so prism = 0.3 3 (3mm decentered each eye ) x 6 =1.8 prism dioptres of unwanted prism in each eye

[mirage2k2]

is the rule any different when the lens also contains prescribed prism. I'm guessing that decentration of a lens containing prescribed prism will generate some induced prism (prentice rule) and also more/less prescribed prism

[ikon44]

is the rule any different when the lens also contains prescribed prism. I'm guessing that decentration of a lens containing prescribed prism will generate some induced prism (prentice rule) and also more/less prescribed prism

you can still use prentices rule,the only difference will be on the amount of prescribed prism you end up with if the PD is incorrect.

[Dave Nelson]

There is no difference beteen "prescribed" prism and prism that is obtained through decentration. Prism can be ground into a lens , obtained through decentration or both. A prism is simply the familiar triangular piece of glass. If you have a flat sheet of glass, there is no refraction, because the sides are parellel. If the sides are still flat, but not parellel, then you have a prism. The apical angle determines the amount of prism diopters, which is based on deviation of a single light ray at a distance of one meter on a flat plane.If the reference plane is an arc, then the resulting measurements is in centrads, not a term used in the ophthalmic industry. This prism, being a triangle, will have both a base and an apex. (hence the "base" orientation in ophthalmic lenses.) If you measure the amount of prism diopters on this prism, you will find there is no change by decentring the lens. The prism amount remains unchanged, therefore prentices rule does not apply. Since power x decentration cm =prism diopters, I can obtain prescribed prism by decentration. Lets say 5 base out. 10x.5=5. The oc must be 5mm different from the pd. But what if the lens is only a -1.00? 1x5=5 This means decentration of 50 mm is needed to obtain the desired prism: an impossibility. The prism will have to be ground, and the lab will grind the lens thicker on one side than the other. The resultant lens will usually be round, but thicker at the prism base than anywhere else on the lens. The person cutting the lens into the frame will put the base as prescribed: base in/out down/up or even at an angle. (compound prism.) I hope this enlightens you a bit. Are you ready to give up engineering and become one of "us?"

is the rule any different when the lens also contains prescribed prism. I'm guessing that decentration of a lens containing prescribed prism will generate some induced prism (prentice rule) and also more/less prescribed prism

[mirage2k2]

Thanks Dave. The whole prism thing is starting to sync in now and I think I'm beginning to understand it all a bit better.

Are you ready to give up engineering and become one of "us?"

The money had better be good :D ... I'm not about to give up my day job ... but there might be opportunities in the future to do some software engineering with "you" :cheers: