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 chodu666
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prsim competency question
Jul 06, 2006 22:27:58 4.5 An understanding of prismatic effect and the manipulation of lens form and setting to obtain the desired control of prismatic effects. hi guys i totally understand vertical differnttial prism and know the options ie 2 sep pairs, slab off, prism controlled etc etc but for normal prescribed prism what would be the method of incorporating prism?? in exam would i say decentration or that its 'worked on'......... is there limits to decentration? and what does 'worked on' mean or entail?? any help appreciated |
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Stephen Meynell
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RE: prsim competency question
Jul 07, 2006 17:19:57 decentering a lens obeys the P=CxF law. So depends on the power of the lens. In practical terms max decentration is limited by the need to have a decent edge substance to fit into the rim. So max decentration rarely is more than 5mm per lens. Which - if given an average power of 3ds - would give a prism of 1.5 dioptres per lens. So max decentration is about 2 to 4 base in out etc per pair of specs. A worked on prism is where the lens is manufactured with a prism on on of its surfaces. It entails setting the surface generator to cut a lens at an angle so the finished lens is thicker at one side than the other. Don't forget stick on fresnel prisms - they can go up quite high! =steve |