Fovea: see a page the way one glance does

At the centre of the retina is a small pit packed with cone cells, the fovea. Its very centre, the foveola, is about a third of a millimetre across and covers roughly one degree of what you see.

Sharpness falls away quickly from there: at two degrees from the centre, you can make out detail only about half as fine. At a laptop’s distance, about 55 centimetres, one degree is about a centimetre on the screen. Do the sums for a 13-inch laptop and the sharpest patch is well under one percent of the screen, roughly one short word of body text.

What lies outside is not simply blurry. The periphery keeps a compressed summary, good for the gist of a scene and for noticing movement, poor for picking out small things among clutter. Letters that are too close together in the periphery run into one another, an effect called crowding, described by Herman Bouma in 1970 and studied closely by Denis Pelli and Katharine Tillman¹. The blur in this tool is a simplification that stands in for it.

The eye does not glide; it jumps. Reading, it pauses for about a quarter of a second, then leaps seven to nine letters on.

The French ophthalmologist Louis Émile Javal gave those leaps their name, saccades, in 1879. In 1901 Raymond Dodge and Thomas Cline photographed the eye’s movements for the first time without touching it. In the 1960s the Russian psychologist Alfred Yarbus showed people Ilya Repin’s painting An Unexpected Visitor and asked them different questions about it: how wealthy the family was, how old they were, how long the visitor had been away. Each question changed where they looked⁶.

So looking depends on what a person is trying to do. On the web, eye tracking by the Nielsen Norman Group found readers often scan text in an F shape⁷, two sweeps across and then down the left, and called it bad for readers and businesses alike, a sign that a page gave them nothing better to hold onto.

Every reader sees a page one small patch at a time. For some, the useful patch is smaller still.

Glaucoma narrows the field from the outside in; macular degeneration damages the centre itself, so people learn to look slightly to one side of what they want to see. With age, the “useful field of view”, how much someone can take in at a glance, shrinks, and in one study older drivers who had lost 40% or more of it were more than twice as likely⁸ to crash. Tiredness, distraction and a cluttered page shrink it for everyone.

This tool changes the pixels you see, so it works on anything in a browser: a live site, a Figma prototype link, a Penpot or Sketch prototype, a Storybook story or a generated app.

Open the prototype, click the bookmark, and click through it. The earlier you catch a problem, the cheaper it is to fix.