A right eye is fitted with a +6.00 DS spectacle lens whose optical centre lies 5 mm temporal to the pupillary axis. Using the Prentice rule, the induced prismatic effect is:
- A 3 prism dioptres, base out ✓
- B 3 prism dioptres, base in
- C 30 prism dioptres, base out
- D 1.2 prism dioptres, base in
Correct answer: A. 3 prism dioptres, base out
Explanation
Prentice rule states prism power in prism dioptres equals decentration in centimetres multiplied by lens power in dioptres: 0.5 x 6 = 3. A convex lens behaves like two prisms joined base to base at its optical centre, so its prismatic base always points toward the centre. With the optical centre displaced temporally relative to the pupil, the base points temporally, giving base out in the right eye. Option C confuses millimetres with centimetres, and option D misapplies the formula.
Reference: Elkington AR, Clinical Optics, 3rd ed.
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Written and medically reviewed by the StethoPrep medical team.