How You See

How you see

Your eye does not deliver a picture to your brain. It makes a small, upside-down, often blurry pattern of light, and the rest is built. Start with the optics: move the letter and watch the lens work.

Plate A model eye looking at a letter F half a meter away. Drag the F toward the eye: the lens thickens to keep the image on the retina, until it reaches the closest distance it can focus, its near point (blue tick). Bring it closer still and the red rays meet behind the retina, and the inset shows the blurred, upside-down F the retina receives. Raise the age to see the near point move away. Distances outside the eye are on a log scale; inside the eye the model is to scale.

Begin: the parts of the eye and how it focuses →

Light from the world passes through a clear window and a flexible lens and lands on a sheet of nerve tissue at the back of your eye. That much is plumbing. What arrives there is a pattern about the size of a postage stamp, upside down, sharp only in the middle, with a hole in it where the optic nerve leaves, sampled by three kinds of color receptor whose signals overlap.

Nothing you see looks like that. The four chapters below follow the pattern from the cornea to the brain and show, at each step, what the eye actually captures and what has to be filled in, inferred or assumed. The assumptions are usually right, which is why you never notice them. Optical illusions are the cases where they are wrong, and that is why they are worth studying.

How this is made and checked

The drawings on this site are computed, not painted. The focusing eye is a thin-lens model from a standard physics text, with the age limits of accommodation from optometry's long-used formula. The rods and cones respond according to measured spectral sensitivities (the Stockman and Sharpe cone fundamentals and the CIE scotopic curve, from the Colour and Vision Research Laboratory database). The color-blindness simulation uses the published matrices of Machado, Oliveira and Fernandes (2009). Every claim is listed with its source, an independent reviewer checks each one, and every simplification is named next to the figure that makes it. The tests here run on an uncalibrated screen, so they demonstrate; they do not screen or diagnose. More about how it is checked.