How You See

Blind spot test

Close your left eye, look at the cross with your right, and move the dot until it disappears. A demonstration, not a medical test.

Hold the screen about the distance you set, square to your face. The test works best with the dot at least two or three hand-widths of screen from the cross; on a phone, turn it sideways or hold it closer.

How to do the test

  1. Set Your eye to the screen to your real viewing distance and keep your head still. Square your face to the screen.
  2. Close your left eye (for the right-eye test) and look steadily at the cross with your right eye. Do not look at the dot.
  3. Drag the dot, or move the slider, slowly away from the cross. When it disappears, press It vanished here; keep going until it returns and press It came back here.
  4. Switch to the left eye: the cross moves to the right and you close your right eye.

On a phone held upright the screen is narrow: the dot needs to reach about 15° from the cross, which at 25 cm is about 7 cm of screen. The tool starts phones at 15 cm for that reason; you can also turn the phone sideways and move it back.

How positions become angles

A browser does not know how large its pixels are: a phone typically draws 150 to 165 CSS pixels per inch, a desktop monitor about 96. So until you calibrate, the tool gives the target's position only as a percentage of the way across the field, and shows no degrees. (It does use a rough estimate of your screen's pixel size to choose a sensible starting distance and position, nothing more.)

To calibrate, open Calibrate the screen, hold a bank or ID card upright against the screen with its short edge, 53.98 mm, along the gray bar, and adjust the bar until it matches. The tool then knows the screen's millimeters, and converts a position x from the cross, seen from your viewing distance D, into a visual angle θ = arctan(x/D).

The result can only be as good as D, which you set by eye. An error of 2 to 3 cm at 30 cm moves an angle near 15° by about 1°, so angles are shown to the nearest degree.

What to expect

The optic disc, where the nerve fibers leave the eye, has no rods or cones[1]. It lies on the nose side of the fovea, about 17 degrees away according to Webvision[2]; in your field of view the blind spot appears on the temple side, roughly 15 degrees out and a little below the line of sight[3], and several degrees across. People's values differ, and your result also depends on calibration and on how still you hold your head.

With Broken line or Pattern selected, notice what you see in the blind region: usually the line or the stripes continue through it. Whether the brain actively fills in the gap or ignores it is still debated[4].

Not a medical test. Everyone has a blind spot, and finding yours says nothing about eye health. Changes in your vision, such as new blind areas, need an eye care professional, not a screen test.

Sources

  1. J. G. Betts, K. A. Young, J. A. Wise and others (2022). Anatomy and Physiology 2e, §14.1 Sensory Perception (Vision). OpenStax, Rice University. textbook
  2. H. Kolb (2011). Simple Anatomy of the Retina (Webvision: The Organization of the Retina and Visual System). University of Utah, John Moran Eye Center. textbook
  3. K. Rohrschneider (2004). Determination of the location of the fovea on the fundus. Investigative Ophthalmology & Visual Science 45(9), 3257–3258. doi:10.1167/iovs.03-1157. paper
  4. R. Gregory and P. Cavanagh (2011). The blind spot. Scholarpedia 6(10). encyclopedia