A 50-year-old emmetrope finds she needs +2.50 D reading glasses to read at 40 cm. Her amplitude of accommodation is now 1.00 D. She asks about multifocal contact lenses. What is the expected near addition required for clear vision at 40 cm, and what is the formula involved?
- A Near addition = object vergence at near minus residual accommodation = 2.50 - 1.00 = +1.50 D insufficient; she needs +2.50 D
- B Near addition = total addition needed at distance
- C Near addition = amplitude of accommodation × 2
- D Near addition = reciprocal of reading distance - available amplitude = (100/40 cm) - 1.00 = 2.50 - 1.00 = +1.50 D add ✓
Explanation
The required near addition for a given working distance is calculated as: required vergence at the near point (in D) = 100/reading distance in cm = 100/40 = 2.50 D. The patient's residual amplitude of accommodation (1.00 D) contributes to this demand. To avoid fatiguing the last remnant of accommodation, half the residual amplitude should be used at most, effectively making the required add approximately 2.50 - 0.50 = 2.00 D. The simplified clinical formula is: Near Add = vergence demand at reading distance - (half the residual amplitude). This is the basis for presbyopia correction and multifocal lens prescribing.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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