This is a 2D phase-space companion to the 3D Helmholtz eye model — same optics, no eyeball. Instead of watching a lens round up in 3D, this view plots the underlying parameter space of the exact equations directly:
Accommodative demand = 1 / u (diopters)
Applied power A = contraction% × A_max(age)
Defocus D = demand − A
Sharp image when |D| < 0.3 D
Left chart — Duane's curve: the age-dependent ceiling Amax(age) plotted directly (10 D at 20, 4–5 D at 40, under 1.5 D past 55). The marker tracks the current age; the applied-power bar shows how much of that ceiling the current contraction is using.
Right chart — defocus field: a 2D grid scanned over every combination of ciliary contraction (x-axis) and target distance (y-axis), each cell independently solved for its own defocus and coloured sharp/mild/heavy. This is a true parameter sweep, recomputed live whenever age changes — not a picture of the eye at all. The bright marker is today's actual slider setting; the boundary between the green sharp band and the surrounding blur band traces exactly where demand crosses the current Amax(age) ceiling, so you can see presbyopia as the green region visibly shrinking toward the "close/high contraction" corner as age increases.
- Age — sets Amax from Duane's curve; shrinks the green sharp band on the right-hand map.
- Ciliary contraction — x-position on the defocus map; how hard the muscle is working right now (0–100% of what it can do).
- Target distance — y-position on the defocus map; sets the accommodative demand 1/u.
- Try to focus — snaps contraction to whatever the target actually needs, capped at 100% — the marker jumps toward (but may not reach) the green band, showing whether that age can meet the demand at all.