A patient looks through a point 5 mm below the optical centre of a -4.00 D spherical spectacle lens. Using Prentice's rule, the prismatic effect experienced by the patient is:
- A 2 prism dioptres, base up
- B 20 prism dioptres, base down
- C 2 prism dioptres, base down ✓
- D 0.8 prism dioptres, base up
Correct answer: C. 2 prism dioptres, base down
Explanation
Prentice's rule states prism power P equals c times F, where c is the decentration in centimetres and F the lens power in dioptres. Here P = 0.5 x 4 = 2 prism dioptres. The base always points toward the thinnest part of the lens, which for a minus lens is its edge, so looking below the optical centre gives base-down prism. Option B confuses centimetres with millimetres, and option D applies the rule to a plus lens direction. This principle underlies prescribed slab-off and decentered lenses.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.