Near Point, Amplitude of Accommodation, and Estimating a Reading Add

For a fully corrected eye, amplitude of accommodation in diopters is 100 ÷ near point in centimeters, so a 12.5 cm near point is 8.00 D. Amplitude falls with age. Ledford’s guideline holds back half the amplitude when estimating a reading add, but the add itself is prescribed by an optometrist or ophthalmologist.

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General study information, not medical advice or a prescription. On the ABO exam this falls under Ophthalmic Optics, 25% of the test, which covers how much accommodation a patient can use and the range of clear vision that results (ABO test specifications). Refraction is 5% of the COA content areas. Both checked Nov 2, 2026.

Accommodation and presbyopia

Accommodation is the eye adding plus power to focus up close. Amplitude shrinks with age; when it no longer covers comfortable reading, that is presbyopia.

Near point to diopters

Measure the near point one eye at a time with the distance correction in place: move small print toward the eye until it blurs. Then amplitude = 100 ÷ near point (cm). Near point 12.5 cm → 8.00 D. Reading distance works the same way: 40 cm needs 100 ÷ 40 = 2.50 D (see vergence).

Amplitude and age

Amplitude drops steadily with age as the lens loses flexibility, so a given reading distance leaves less focusing in reserve each decade. Published age tables differ because they were measured in different ways, which is why a measured amplitude beats a table value. The accommodation calculator shows one set of age estimates for practice.

Working distance and add (estimate, not a prescription)

Ledford’s guideline is to hold half of the amplitude back in reserve, because giving the most plus a patient will accept wipes out their own focusing at near. As arithmetic: add ≈ reading demand − amplitude ÷ 2. Example: reading at 40 cm (2.50 D) with 2.00 D amplitude → 2.50 − 1.00 = +1.50 D. The real add comes from the prescriber’s refraction.

Why opticians care

Knowing the working distance behind an add helps when choosing multifocal designs, segment heights, and occupational or computer lenses, and when a patient says the reading zone is too close or too far.

Practice problems

1. Near point 20 cm. Amplitude?

   Show answer

   100 ÷ 20 = 5.00 D

2. Near point 33⅓ cm. Amplitude?

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   100 ÷ 33.3 = 3.00 D

3. Reading at 40 cm with 2.00 D amplitude, half kept in reserve (Ledford’s rule). Estimated add?

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   Demand 2.50 − (2.00 ÷ 2) = +1.50 D (teaching estimate)

4. Reading at 33⅓ cm with 3.00 D amplitude, half kept in reserve (Ledford’s rule). Estimated add?

   Show answer

   Demand 3.00 − (3.00 ÷ 2) = +1.50 D (teaching estimate)