Self-screening · ~3 min

Visual acuity check.

A Snellen-style letter test that estimates how sharp your distance vision is, one eye at a time. Results are a relative estimate for education and awareness — not a certified measurement.

How to do this test

Sit upright, at a consistent distance from your screen — roughly 1.5–2 m (5–6.5 ft) from a computer monitor, or arm's length or farther if you're on a phone or tablet.
Use a well-lit room and turn your screen brightness up, with no glare on the display.
Keep your usual glasses or contacts on, unless you're deliberately checking uncorrected vision.
Cover the left eye first (a clean palm or card works — don't press on the eye) and test the right eye.
For each line, type or click the 5 letters you see, then submit. Use "Can't read this line" if it's too blurry to guess.
Switch eyes and repeat, then review your per-eye estimate.

Before you start

This screening uses your screen's on-page size, not a physically measured chart, so the 20/xx label it gives you is a relative estimate, not a clinically certified visual acuity. True 20/20 measurement needs a calibrated chart at a fixed, measured distance — the kind an optometrist uses in an exam room.

Sit at a steady, comfortable distance — about 1.5–2 m from a monitor, or arm's length or more from a phone/tablet.
Good lighting, low glare, brightness turned up.
You'll test one eye at a time, starting with the right.

Clinical background

The science behind this

Visual acuity is the eye's spatial resolving power — the finest detail your optics, retina, and visual pathway can distinguish. The Snellen fraction expresses it as a ratio: 20/40 means you resolve at 20 feet what a typical eye resolves at 40.

Standardized optotypes are built on angles, not arbitrary sizes. A letter on the 20/20 line subtends 5 arcminutes at the testing distance, with each stroke and gap just 1 arcminute wide — right at the resolution limit set by the spacing of foveal cone photoreceptors and the eye's optics.

What it screens for

Uncorrected refractive error (myopia, hyperopia, astigmatism)
Amblyopia and other causes of reduced acuity in one eye
Media changes that blur the image, such as cataract

Limitations

A browser can't know your screen's true pixel size or your exact distance, so results are relative, not a calibrated 20/20.
Lighting, contrast, and display resolution all shift the outcome.
It flags reduced acuity but can't identify the cause — that needs a refraction and exam.