Vision Screening with a Smartphone, Bounded by Optics: A Protocol Derived from Display, Camera and Pinhole Limits

A smartphone screening protocol derived from the physics of the phone itself. Display pixel subtense across 326-500 PPI shows that at 2 m every phone renders a 6/6 optotype stroke with at least 7.47 pixels, while at 40 cm the lowest-density phone robustly renders only 6/15. Eccentric photorefraction with a 10 mm built-in flash separation at 1 m has a geometric dead zone of 3.33 dioptres and a smallest reliably detectable crescent at 4.69 dioptres, so it can flag only very large errors. Acuity plus a 1.2 mm pinhole detects defocus from 0.63 dioptres at the 6/12 boundary and assigns a refractive cause from 1.19 dioptres. The specified protocol (2 m distance acuity, 6/12 referral threshold, pinhole step, gross photorefraction flag only) has modelled sensitivity 95.18 percent and specificity 98.82 percent under stated population and noise assumptions. Fine acuity grades, dioptre values, astigmatism axis and internal eye health are explicitly outside the supported limits. SIMULATED, NOT MEASURED.