Fovea

At the center of the retina, the light-sensing back of the eye, is a small pit packed with the cells that see color and detail. That pit is the fovea. Its very center, the foveola, is about a third of a millimeter across and covers roughly one degree of what you see.

Sharpness falls away quickly from there: at two degrees from the center, you can make out detail only about half as fine. At a laptop’s distance, about 55 centimeters, one degree is about a centimeter 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. That is your thumbnail.

What lies outside is not simply blurry. The edge of vision keeps a rough 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 doesn’t glide across a page; 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 where the thumbnail lands 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 customer sees your page one small patch at a time. For some, the patch is smaller still.

Glaucoma narrows the field from the outside in; macular degeneration damages the center 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.