The lens and the diaphragm in the camera obscura

How it began

For centuries the camera obscura used a bare hole, which gives a dim, soft image. In 1550 Gerolamo Cardano wrote of putting a glass disc in the opening. In 1568 the Venetian Daniele Barbaro described using an old man’s spectacle lens, and found that covering part of it so that only a small opening remained made the image sharper.

How it works

A lens gathers light from a much larger opening than a pinhole and brings it to a focus, so the image is far brighter. But a simple lens is sharp only near its centre. Making the opening smaller, which we now call stopping down or closing the aperture, cuts out the edge rays that blur the image, at the cost of light.

Why it mattered

Barbaro’s trade-off between brightness and sharpness is the basic choice every photographer still makes. With a lens, the camera obscura became a practical drawing aid for artists, and the device Niépce and Daguerre would later fit with light-sensitive plates. See light and the pinhole.

Limits and what replaced it

Single spectacle lenses had strong aberrations and fringes of colour. Better lenses, designed by calculation from the nineteenth century, replaced them.

Today

Every camera still pairs a lens with an adjustable aperture, the combination these Renaissance writers described.

Sources

• Wikipedia: Camera obscura ↗
• Eder, ch. V (p. 36)