Catch the light. Find the focus.: sources & model

Open a sourced 3D eye, move a target nearer, and catch the moment its light comes together. Then discover why a smaller pupil is different from better focus.

What supports the explanation?

What this model assumes

1. Selected BodyParts3D 4.0 gross anatomy, with teaching colors, clipping and optional exploded positions. Source-linked mesh provenance and modifications are available. No patient-specific anatomy or measured lens deformation.
2. Separate monochromatic paraxial optical model with one equivalent surface and a planar retinal target. Optical coordinates are not anatomical mesh measurements.
3. Target range 20–200 cm, plus a true infinity setting. Distant top/bottom points use incident angles ∓0.02 rad; finite top/bottom points are ±10 mm from the axis. Diagram axes are independently enlarged and the incoming distance is compressed.
4. Accommodation is clamped to 0–capacity. Ideal correction is co-located with the equivalent surface. No negative accommodation, hidden retinal movement, spectacle vertex distance or clinical prescription.
5. Point footprints are sampled ray intersections before neural processing. Their plotted scale is stated. No acuity, color vision, diffraction, aberrations, eye movements, binocular depth or disease simulation.
6. Animation markers reveal ray direction slowly; they do not show the speed of light. The anatomical model itself is static. Independent subject and learner review remains pending.
7. Anatomy and a useful reduction: The anatomy view preserves selected BodyParts3D eye meshes in shared coordinates. The focus bench uses a separate reduced eye: one equivalent powered surface, refractive index 4/3 inside, relaxed power 60 D, and a retinal target 22.222 mm from that surface. That distance is an optical coordinate, not a measurement of this anatomical mesh. The equivalent surface represents the combined optics, not just the cornea.
8. Diopters measure optical power: One diopter is one inverse meter. In this paraxial model, incoming vergence is −1/u for a target u meters away. Add the model eye power and ideal correction: L′ = −1/u + 60 + A + C. Image distance is v = (4/3)/L′. At 25 cm, incoming vergence is −4 D; accommodation A = 4 D restores the baseline focus.
9. Trace a bundle, not a single magic ray: For each object point, we calculate rays through many aperture positions. At focus they meet at one image point. A point above the axis maps below it: the optical image is inverted. Covering half the aperture removes some rays from every visible point; it does not remove half of the object.
10. A footprint is not an eyesight score: With full aperture diameter Dp and focus distance v, the on-axis geometric footprint at retinal distance r is b = Dp |1 − r/v|. Halving pupil diameter halves that defocus diameter and quarters admitted light, before the half-cover option. Diffraction, aberrations, receptor sampling and perception are omitted. The smallest pupil is not universally the best eye.
11. Two different limits: A longer model retinal distance can put distant focus in front of the target. Negative ideal correction can move focus back without shrinking the anatomy. Separately, limited accommodation can prevent near focus even with baseline geometry. These illustrate refractive mismatch and accommodation range; they do not provide an eyeglass prescription or a personal diagnosis.

What has been checked

Analytical reference cases, conservation or transition invariants, finite drawing commands, bounded setup parsing, discovery and route integrity are checked automatically. These checks do not establish anatomical fidelity, learner outcomes or browser/device compatibility. Independent subject review, learner trials, comprehensive accessibility review and browser video encoding checks remain pending.

Each source supports the associated claim. Sources do not certify this implementation or its visuals.

Download the eye geometry provenance · CC Attribution 4.0 International

About the cover illustration

The cover is a rendering of selected BodyParts3D eye meshes, with an inspection cut and teaching colors. BodyParts3D, © The Database Center for Life Science licensed under CC Attribution 4.0 International. Selection and modifications are recorded in the eye model’s downloadable provenance manifest.