The camera obscura is based on a simple principle. If you go into a dark room (thus the name, the Latin camera, "room", and obscura, "dark") and punch a small hole in the wall, the image outside will be projected inside. Francis Bacon understood the apparatus; Da Vinci described them in his notebooks; Frisius and Kepler used camera obscura projections to help perform their observations of the sun. The camera obscura became a staple of Victorian seaside resorts; one built in the 1940s at Cliff House in San Francisco provides a pleasant view of the elephant seal rocks. Recently, one controversial book suggested that Dutch master Johannes Vermeer may have used the camera obscura in his art. Certain hints of the perspective Vermeer used, the physical evidence suggesting that Vermeer was able to very accurately render objects’ proportionally without measuring them, the apparent finding that many of Vermeer’s paintings were made in the same room, and a tantalizing question of whether one object in a painting represents Vermeer’s darkened booth all piqued architecht Philip Steadman‘s interest. The question isn’t settled, and it may never be. For one thing, why wasn’t Vermeer’s lens, which would have been a rare and quite valuable item, recorded in his effects when he died? Vermeer, though a master painter and member of Delft’s painters’ guild, was primarily an art dealer, and could quite likely have afforded it, but would it have vanished out of history? But Steadman’s thesis is nothing compared to that of painter and photographer David Hockney.

Hockney, a major British artist, had been puzzling over portraiture of Jean-Auguste-Dominique Ingres. How did Ingres’ small — often 12 inches by 18 inches — and quickly drawn portraits have such a confident line? Then, he recounted in a New Yorker article on his theory, it hit him: "[O]ne morning, studying the blowups, I found myself thinking, Wait, I’ve seen that line before. Where have I seen that line? And suddenly I realized, That’s Andy Warhol’s line." Hockney thought that Ingres had been quickly tracing a projected image, as Warhol did on many of his paintings. Eventually Hockney came to the conclusion that Ingres had been using a camera lucida ("bright room") to set down quickly a sketch of key points of the face, and had been concealing his use of the device. An artist might hide his use of mechanical aids; the nineteenth century American realist painter Thomas Eakins worked from photographs and took some pains to conceal the fact.

Hockney worked with the camera lucida, a sort of prism on a stick, to demonstrate that the technique was viable. Ross Woodrow, a lecturer in art at Australia’s University of Newcastle, strongly disputes Hockney’s take on things (and even what Hockney asserts is the pseudo-photographic nature of his portraits), but this is a viable approach to demonstrating that a wild theory is at least a practical wild theory. Thor Heyerdahl may not have proved anything about Polynesian sailing, but at least he demonstrated that his idea would have worked. Then, however, Hockney got religion. He started seeing what he felt were unmistakable signs of the use of advanced optics everywhere; in van Ecyk’s work, in Holbein’s The Ambassadors (with its anamorphic skull), in the Mona Lisa. He began to assert that the entire Renaissance revolution in painting, the improvement in technique that occured after around 1420, could best be understood as a revolution in optics. Painters in the Renaissance didn’t necessarily understand perspective better, Hockney argued; they simply had access to concave mirrors that they could use to project images onto the canvas.

The French biographer and perspective theorist Jean-Francois Niceron (whose work Marcel Duchamp claimed inspired his own), roughly a contemporary of Vermeer’s, had mused about using the camera obscura as an artist’s tool. Vermeer lived near master lensmaker Antony von Leeuwenhoek. Vermeer was a single painter; if he had stumbled across an innovative trick using lenses, he might have been able to keep it hidden. Hockney was forthrightly proposing that there had been centuries of conspiracy, that Western European painters had passed a secret from one generation to the next while never revealing it to the outside world; his book making the argument was called Secret Knowledge. It was enormously controversial to art historians; a conference on his claims (dubbed a "smackdown" by ArtKrush magazine) largely featured critics. But art historians weren’t simply upset that they had missed the single most important fact about the Renaissance; in addition to the fact that Hockney’s theory relied upon two hundred years of artists keeping a secret from the outside world, in the face of strongly-worded doubts about some of the internal evidence Hockney finds in the paintings he studied, a complete lack of documentary evidence (Why haven’t any of these mirror apparatuses ever turned up? Leonardo da Vinci recorded his musings about the camera obscura; why wouldn’t he have noted its use in painting in his secret notebooks? Why didn’t any sitters — or their highly-informed secretaries — ever mention the use of mirrors in letters?) or even the scant physical evidence suggesting Vermeer’s use of the camera obscura, Stanford professor David Stork noted that the convex mirrors didn’t work. Mirror-making, like lens-making, simply wasn’t advanced enough in the 1400s to provide the features Hockney claimed he saw in nearly every Renaissance master’s work. Some of the oddities Hockney points to are compellingly weird; it seems likely that at least some painters used optical techniques more than we realized. But Hockney has become a conspiracy theorist, and at some point, most conspiracy theorists go beyond mere fact. They’ve seen the truth, and everything, even random noise or the absence of evidence, is another data point proving their claim. True believers don’t have to think about the null hypothesis that there wasn’t a two hundred year tradition of Dutch and Italian painters (painters, that close-mouthed and conspiracy-minded cabal!) using devices a hundred years too advanced for their time. He’s shining a light into the darkness.

"I was talking with another historian the other day, and he assured me that no left-handed person would ever have been allowed to become pope in those days: the left was the devil’s hand. Sinistra. But that’s the effect you would get, in the early days of lens projection, if you hadn’t yet learned to compensate for the reversal caused by the lens. For that matter, look through the rest of the book: Lorenzo Lotto’s ‘Man with a Golden Paw’; he, too, appears to be holding the object in his left hand. Doesn’t it seem to you there are an inordinate number of left-handed people in this book?" He paused again before positively exulting, "I’m right. I’m right. I’m more certain of it every day."