Near Add
Optics and Optometry
The reading addition for a working distance, keeping half the amplitude in reserve.
How it is calculated
- The accommodative demand of a working distance is its reciprocal in metres: 40 cm needs 2.50 D, 33 cm needs 3.00 D.
- The add is that demand minus half the patient's amplitude of accommodation.
- Half the amplitude is deliberately left unused. Sustained near work at more than half of maximum effort is what produces asthenopia, so the reserve is the comfort margin rather than a safety factor.
- The result is rounded up to the next 0.25 D step, because under-adding leaves the patient short of the distance they asked for.
- A measured amplitude is always better than an age estimate. Where only an age is given, Hofstetter's expected value is used and the tool says so.
- The add should always be checked against the patient's own range of clear vision before it is prescribed.
References
- Standard optometric practice — the half-amplitude-in-reserve rule.
- Hofstetter HW. Useful age-amplitude formula. Optom World 1950;38:42-45.
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