Blind Spot Test

What it measures

Where your eye's natural (physiological) blind spot sits — the small region of each eye's visual field with no vision because the optic nerve leaves the retina there and that patch has no light-sensing cells. Everyone has one in each eye, roughly 12-18 degrees to the outer (temporal) side of where you are looking, and your brain normally fills it in so you never notice it.

How it works

First you calibrate by sizing an on-screen card to a real bank card (85.6 mm wide), which tells us how many pixels make a millimetre, and you set how far you sit from the screen. Then, for one eye at a time, you cover the other eye and stare at a + marker while a small dot sits off to the outer side. You slide the dot away from the + until it suddenly disappears — that is the moment its image lands on your optic disc. From the on-screen distance and your viewing distance we estimate the angle, in degrees, at which your blind spot lies.

Tips for an accurate result

• 1Stare, don't chase — The illusion only works if your gaze stays fixed on the +. If you look at the dot it will never vanish.
• 2Sweep slowly around 12-18 degrees — Move the slider gently back and forth in that range; the dot pops out of view across a narrow band.
• 3Move a little closer if needed — Sitting slightly closer enlarges the on-screen geometry and can make the disappearance easier to catch.
• 4A new or fixed dark patch is different — Your physiological blind spot is invisible in daily life. A spot, shadow, curtain or missing area you can actually notice is not this — get it checked promptly.