A patient wearing a +6.00 D spherical lens reads with his visual axis passing 5 mm below the optical centre of the lens. Using Prentice's rule, the prismatic effect induced is:
- A 3 prism dioptres, base down
- B 3 prism dioptres, base up ✓
- C 0.3 prism dioptres, base up
- D 30 prism dioptres, base down
Correct answer: B. 3 prism dioptres, base up
Explanation
Prentice's rule states that prismatic effect in prism dioptres equals the distance from the optical centre in centimetres multiplied by the lens power in dioptres. Here, 0.5 cm multiplied by 6.00 D gives 3 prism dioptres. In a convex lens the prism base always points toward the thicker portion, which is toward the optical centre; looking below the centre therefore gives base-up prismatic displacement. Option C reflects a decimal error and option D a unit error.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.