Hyperacuity

VERNIER
ACUITY
TEST

Two short vertical lines, one above the other. The lower one is nudged a hair left or right. Your job: call the direction. Humans are astonishingly good at this — we detect misalignments finer than a single cone in the eye. It's called hyperacuity, and this finds your limit.

How it works

Left or right?

Say which way the bottom line is shifted. Guess if you truly can't tell.

It gets harder as you nail it

Two right in a row shrinks the offset; one miss widens it. This homes in on your threshold.

~24 trials

The offset where you're right about 71% of the time is your acuity.

Screen calibration — hold any bank card flat against the screen

MATCH THIS BOX
TO YOUR CARD'S WIDTH

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Not calibrated — assuming a 96 dpi desktop screen

Every card on Earth is 85.6 mm wide. Without this the result is only correct on an unscaled desktop monitor — on a phone it overstates by more than half.

Viewing distance from screen (cm)

Used only to convert your result into an angle. ~50 cm is typical. A rough number is fine.

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TRIAL 1 / 24—

Which way is the lower line shifted?

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Results

YOUR
HYPERACUITY

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Note

This measures vernier (alignment) acuity, which is not the same as letter-chart sharpness. The pixel threshold is measured directly; the arc-second angle is estimated from the distance you entered and an assumed screen pixel size, so it's approximate, not clinical. Anti-aliasing lets browsers draw sub-pixel offsets, which is exactly what makes this possible. Not a vision-health assessment.

About the vernier acuity test

Vernier acuity is a hyperacuity — the ability to detect that two line segments are misaligned, at a scale finer than a single cone in your retina. This test shows two short vertical lines with the lower one nudged a hair left or right; you call the direction, and a two-alternative staircase narrows the offset until you reach your limit.

The pixel offset at your threshold is measured directly; the arc-second figure is estimated from the viewing distance you enter and an assumed screen pixel size, so it's approximate. What makes sub-pixel offsets possible at all is anti-aliasing, which lets the browser draw a line between pixels — your visual system then averages across receptors to resolve alignment below the pixel grid. It's a genuinely different measure from letter-chart sharpness.

Frequently asked questions

What is vernier acuity?

The ability to detect a small misalignment between line segments. It's called a hyperacuity because people resolve offsets far finer than the spacing of the cones sampling the image.

How is it different from 20/20 acuity?

Letter acuity measures the smallest detail you can resolve; vernier measures the smallest offset you can detect — typically many times finer.

Are the arc-second numbers exact?

The pixel threshold is measured directly, but the arc-second conversion depends on the distance you entered and an assumed pixel size, so treat it as an estimate.

How can I detect an offset smaller than one pixel?

Anti-aliasing renders lines at sub-pixel positions, and your visual system averages across many receptors, letting you resolve alignment below the physical pixel spacing.

VERNIERACUITYTEST

How it works

YOURHYPERACUITY

About the vernier acuity test

Frequently asked questions

VERNIER
ACUITY
TEST

YOUR
HYPERACUITY