Ciliary Muscle & Lens Accommodation

Focusing effort

Patient age 40 yr

Ciliary muscle contraction 50%

Target distance 63 cm

Live readouts

Max amplitude (age)

6.0 D

Near point

16.7 cm

Demand at target

1.6 D

Power applied

3.0 D

Defocus

-1.4 D

Retinal image

Sharp

How it works

Focusing on something near is the Helmholtz mechanism: the ciliary muscle contracts, which slackens the zonule fibers holding the lens equator, and the lens's own elastic capsule lets it round up — increasing its curvature and optical power. Relaxing the muscle re-tensions the zonules and flattens the lens back down.

Required power is a vergence sum. For an eye resting at power P_rest (focused at infinity) viewing an object at distance u (metres):

Accommodative demand  =  1 / u        (diopters)
Outgoing vergence  V' = -1/u + P_rest + A
Sharp image when   V' = P_rest,  i.e. applied accommodation A = 1/u

The lens can only round up so far. Its own tissue stiffens with age even though the ciliary muscle keeps contracting normally — so the maximum accommodative amplitude A_max declines steadily (Duane's curve): roughly 10 D at 20, 4–5 D at 40, and under 1.5 D past 55. That decline, not any muscle weakness, is presbyopia. The near point — the closest sharp-focus distance — follows directly: near point ≈ 1 / A_max.

Age — sets A_max from Duane's curve; the ciliary ring still contracts fully at any age, but the lens (thickness in the 3D view) responds less.
Ciliary contraction — how hard the muscle is working right now (0–100% of what it can do).
Target distance — sets the accommodative demand 1/u; the light-ray fan shows whether it converges exactly on the retina (sharp) or in front of/behind it (blur).
Try to focus — snaps contraction to whatever the target actually needs, capped at 100% — showing whether that age can meet the demand at all.

Crystalline lens Ciliary muscle ring Light rays / focus point

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