American Board of Opticianry (ABO) Practice Test

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A -5.00 lens must be decentered by how many millimeters to induce 2 diopters of base-in prism?

  1. 2mm

  2. 3mm

  3. 4mm

  4. 5mm

The correct answer is: 4mm

To induce prism in a lens, it needs to be decentered. Prism power is directly proportional to the amount of decentration. In this case, a -5.00 lens requires 4mm of decentration to induce 2 diopters of base-in prism. This is because 1 diopter of prism is induced for every millimeter of decentration for a -5.00 lens. Therefore, for 2 diopters, the lens must be decentered by 2mm + 2mm = 4mm.

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