Accommodative Amplitude: Why Close Work Tires Some Eyes and Not Others

October 20, 2026 · 8 min read

"I can read it. I just cannot read it for long."

That sentence separates two different problems. Being unable to read at 40 cm is a question of optical power. Being able to read for five minutes and then needing to stop is a question of stamina, and stamina in the eye has a measurable basis. The relevant quantities are the amplitude of accommodation, the accommodative response, and accommodative facility. They are not the same thing, and a normal value in one does not guarantee the others.

What accommodation is, mechanically

Inside the eye sits a crystalline lens that is flexible in youth and progressively stiffer with age. In the unaccommodated state the ciliary muscle is relaxed and the zonular fibres are under tension, which flattens the lens. When the ciliary muscle contracts, the ring it forms gets smaller, zonular tension drops, and the lens bulges into a more convex, higher-powered shape. That increase in power is accommodation, and it lets a single eye focus from a distant object to a near one.

The range over which this works is bounded by two points. The far point is where the relaxed eye focuses, and the near point is the closest distance at which the eye can still hold a clear image. The distance between them, expressed in dioptres, is the amplitude of accommodation. Near point at 10 cm means 10 dioptres of amplitude; near point at 20 cm means 5 dioptres. That conversion is the only arithmetic needed: amplitude equals 1 divided by the near point distance in metres.

Putting an expected number on it

Amplitude falls predictably with age, and Hofstetter's formulas are the standard way to state what is expected. The minimum expected amplitude is 15 minus 0.25 times age in years; the average is 18.5 minus 0.3 times age; the maximum is 25 minus 0.4 times age. All in dioptres.

• At age 20: minimum 10.00 D, average 12.50 D.
• At age 40: minimum 5.00 D, average 6.50 D.
• At age 50: minimum 2.50 D, average 3.50 D.

The formulas describe populations, not individuals, and they are useful mainly for spotting someone who is far below the expected band. A 45-year-old with 1.50 D of amplitude is not merely "getting older"; that is below the minimum, and it changes what a pair of glasses needs to do.

The other number that matters is demand. Reading at 40 cm requires 2.50 D of accommodation, because 1 divided by 0.40 m is 2.50. Reading at 25 cm requires 4.00 D. A clinical rule of thumb is that sustained comfortable near work uses no more than about half of the available amplitude, so a person with 5.00 D can comfortably hold 2.50 D of demand. Treat that halving rule as a heuristic rather than a law: it is a convention for estimating when symptoms are likely, not a measured threshold.

Amplitude is not the whole story

Two people can both measure 5.00 D of amplitude and have very different experiences of reading, because amplitude describes the range, not the control. Three other quantities fill the gap:

• Accommodative lag. The eye rarely focuses exactly on the target. Lag is the gap between the demand and the response, typically a fraction of a dioptre in a comfortable reader and larger in people who report fatigue.
• Accommodative facility. How quickly the eye can shift focus between two distances, usually measured by counting cycles of alternating plus and minus lenses in a minute. Someone with good amplitude but poor facility struggles with switching between a screen and a document.
• Convergence coupling. Focusing and converging are linked, so a near task loads both systems. This is why an eye problem can present as a reading problem and vice versa.

Why close work tires the eye

Sustained accommodation is only part of the load. Reading also suppresses blinking, which destabilises the tear film, and that alone produces fluctuating clarity and a gritty feeling after twenty minutes. Convergence adds a second muscular demand. The honest position is that "eye strain" from close work is usually a combination of accommodative demand, tear film instability and reduced blink rate, and the relative weight varies by person. That is why simply adding plus power sometimes helps and sometimes does nothing.

Four claims that do not survive scrutiny

• "Eye exercises will restore your amplitude." Presbyopia is driven by lens stiffness and capsular changes, not by a weak muscle, so no exercise restores lost amplitude. Vision therapy has a legitimate role in convergence insufficiency, which is a different diagnosis.
• "If I can read at arm's length, my eyes are fine." Amplitude says nothing about lag or facility. Plenty of people pass a near point test and still cannot work at a screen for an hour.
• "Reading glasses weaken your eyes." Readers do not reduce amplitude; they replace the accommodation the lens can no longer supply. Not wearing them does not preserve anything.
• "A small plus add is always harmless." Add power reduces the range of clear distance vision, and an unnecessary add can make the world soft beyond a metre. It is a trade, not a free improvement.

When tired eyes deserve an examination

Book an eye examination if your near vision has changed noticeably over months rather than years, if the fatigue is one-sided, if you see double at near, if you need to hold material further away than you did a year ago, or if headaches cluster after reading and not after other tasks. Ask specifically for amplitude, accommodative response and facility to be measured, not just a refraction. Those tests take a few minutes and they answer a different question from "what is my prescription" — which is exactly why a normal prescription can coexist with a real problem.

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