Lazy Eye Test (Amblyopia)

What it measures

This screen estimates and compares crowded-letter acuity between your two eyes, looking for a meaningful interocular difference that can be a sign of amblyopia (commonly called 'lazy eye') — reduced vision in one eye that did not develop normally. Because amblyopic eyes are disproportionately hurt by surrounding ('crowding') letters, each target is shown flanked by other letters to make any difference easier to surface.

How it works

You cover one eye fully and read a descending ladder of single Sloan letters, each surrounded by flanking letters at the same size and a small gap — the 'crowding' manipulation that amblyopia is especially sensitive to. For each line you pick the central target from four choices, or say you can't read it. The engine records the smallest line you read correctly for each eye, then compares the two as a number of lines on the chart. A clear, repeatable two-line-or-more gap between the eyes — particularly with crowding — is the kind of difference worth checking with a professional eye exam. Letter sizes are derived from your card calibration and viewing distance, so they reflect real angular size on your display.

Tips for an accurate result

• 1Test the same conditions for both eyes — Same distance, same lighting, same correction. Any difference you find is only meaningful if everything except the eye stays constant.
• 2Don't memorise the letters — If you read the right eye's lines first, the target letters you remember can flatter the left eye. Answer each line on what you actually see, not what you recall.
• 3Have children's eyes checked early — Amblyopia is a childhood condition where early treatment matters most. A home screen is no substitute for a children's eye check — book a professional exam if you have any doubt about a child's vision.