Kepler's optics
1604 (Ad Vitellionem paralipomena); 1611 (Dioptrice)
Johannes Kepler explained how the eye forms an upside-down image on the retina, used the name camera obscura, and in 1611 worked out how lenses form images.
How it began
Kepler came to optics through astronomy: he wanted to know why pinhole images of the Sun during an eclipse looked the size they did. In 1604 he published a book of optics that built on Ibn al-Haytham and the medieval writer Witelo, and used the name camera obscura, “dark chamber”, for the device.
How it works
Kepler showed that the eye’s lens throws an inverted image onto the retina, exactly as a lens throws one onto the back wall of a camera obscura. In his 1611 Dioptrice he traced rays through convex and concave lenses and explained how a telescope magnifies.
Why it mattered
He made the camera a model of vision and laid the ground for designing lenses by geometry rather than trial. Portable camera obscuras, including tents and boxes, followed.
Limits and what replaced it
Kepler did not have the exact law of refraction. Snell’s law, found a few years later, made precise lens calculations possible.
Today
The picture of the eye as a camera, and the ray optics used to design lenses, both go back to Kepler.