How the simulation works, and its limits
The refraction is split into its two principal meridians. After subtracting any accommodation, each meridian's leftover defocus (in diopters) spreads a point of light into a line whose angular length is roughly pupil diameter × defocus. Both meridians together make an elliptical blur patch, which is convolved with the image using WebGL. Sample weights follow the Stiles–Crawford effect. Even so, geometric optics overstates perceived blur (no diffraction or higher-order aberrations, and no neural adaptation), so the blur is drawn at a scale of 0.7 by default; the "20/xx" figure comes from an empirical fit to uncorrected-acuity data rather than from the picture. For an infant or toddler the retina and visual cortex are still maturing, so even a perfectly corrected eye does not see 20/20 yet.