Vernier acuity is the ability to tell that two line segments are not collinear. It is called a hyperacuity because the threshold is finer than the spacing of the photoreceptor mosaic itself — the retina does not resolve the offset, the cortex infers it from the luminance distribution across several receptors. Reported thresholds sit around 2–5 arcseconds, roughly ten times finer than standard visual acuity, which is about one arcminute, or 60 arcseconds.
That sensitivity is precisely why it is clinically useful. Metamorphopsia — the perceived warping of straight lines caused by fluid lifting the macula — shows up in a hyperacuity task long before it shows up on a letter chart. Preferential hyperacuity perimetry exploits exactly this, and has been reported discriminating wet AMD with an area under the ROC curve of 0.87.
Now put that on a screen. At a 60 cm viewing distance, a single pixel on a typical display subtends roughly 90 arcseconds. The quantity being measured is 2–5. One pixel is about twenty times coarser than the entire clinical range.
The demonstration above still reaches sub-pixel thresholds, and the reason is worth stating: an offset of a fraction of a pixel is not discarded, it is antialiased. The renderer distributes the line's energy across neighbouring pixels, shifting the luminance centroid by less than the pixel pitch, and the visual system integrates that shift. So the task is measurable — but below one pixel you are partly characterising the rendering pipeline, the panel's subpixel geometry and its gamma, not only the observer.
This is the distinction between a demonstration and an instrument. A browser can show you the phenomenon and let you feel where your own threshold lies. It cannot control viewing distance, luminance, contrast calibration, refraction, or pixel geometry — and without those the number is not a measurement of the eye. That is not a disclaimer bolted onto the end of a product. It is the reason the product was labelled not for diagnostic use, and it is the kind of constraint that has to be settled before any code is written, not after.
Sources —
The Clinical Use of Vernier Acuity: Resolution of the Visual Cortex Is More Than Meets the Eye ·
Accuracy of a Self-monitoring Test for Identification and Monitoring of Age-related Macular Degeneration