Did the Old Masters secretly trace their paintings with lenses and mirrors?

Probably not in the sweeping way the theory claims, though some painters certainly used optical devices. The 'Hockney–Falco thesis', proposed by the painter David Hockney in 1999 and developed with the physicist Charles Falco, argues that the leap in realism of Western art from the early Renaissance on was mainly due to optical aids such as concave mirrors, lenses and the camera obscura, which project an image a painter could trace. Most art historians reject that strong version, while accepting that individual artists, especially after about 1550, did use optics.

It began with a hunch. In 1999, Hockney noticed that tiny portrait drawings by Ingres looked like Andy Warhol's drawings traced from photographs. He then lined up images of Western art by date on a 'Great Wall' in his studio, and saw what looked like a sudden jump in naturalism around 1420. Falco, an optics expert, studied a rug in a painting by Lorenzo Lotto that seems to slip out of focus, as if Lotto had refocused a lens. A concave mirror built to their calculations did project a usable image.

The critics hit back hard. Realist painters, then and now, achieve similar accuracy by eye: one researcher had an artist paint a chandelier like van Eyck's freehand, with similar accuracy. Another optics expert found none of the distortions a mirror or lens should leave. And historians point out that the whole case rests on a few dozen square centimetres of canvas.