Projecting an image

Did the Old Masters paint using optical projection techniques?

Painters from Jan van Eyck to Jean-Auguste-Dominique Ingres may have achieved a remarkable imitation of nature not through sheer painterly talent, but by using optical devices, according to David Hockney's controversial recent book Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters (Penguin Putnam, 2001). Specifically, Hockney, assisted by optical scientist Charles Falco of the University of Arizona, argues that from around 1430 many painters used a concave mirror to project brightly lit subjects onto a canvas, allowing them to render figures with unprecedented naturalism. Later, at around the end of the sixteenth century, according to Hockney and Falco, painters including Caravaggio began to use refractive lenses instead of concave mirrors to project their images for tracing (see Nature 412, 860; 2001).

Since the publication of Hockney's provocative thesis, several objections have been raised. For example, it has been claimed that Hockney's projection technique would have required a mirror of very long focal length to create an image suitable for tracing, and that this would exceeded the technical capabilities of Renaissance mirror-makers.

New evidence concerning the history of optical projection further challenges Hockney's hypothesis. In particular, a close reading of the historical documents suggests that the specific device that Hockney and Falco claim was used widely by artists from the 1430s was in fact invented by the Neapolitan magician Giambattista della Porta in 1558, and then rendered obsolete by della Porta himself in 1589.

Della Porta was as famous for his investigations of arcane natural processes and mechanical contrivances as for being a playwright and impresario. One of the many instruments that intrigued him was the camera obscura — the generic name given to the projection of inverted images through a small hole into a darkened chamber. In 1558, he gave the earliest description of a new type of camera obscura in the first edition of his widely read book Natural Magic. The new technique involved using a concave mirror to project an inverted image onto a piece of paper. This is the first documented account of the device that Hockney and Falco claim was used by artists from the 1430s.

Incidentally, the first account of incorporating a convex lens into the camera obscura dates from just eight years earlier, in the encyclopedic work of the astrologer and mathematician Girolamo Cardano called On Subtlety, which also describes in detail the workshop techniques of contemporary painters.

In the expanded second edition of Natural Magic, published in 1589, della Porta added a dramatic revision to his concave-mirror camera obscura. As a result of his extensive investigations of optical instrumentation on the Venetian glass-making island of Murano in 1580, he combined a convex lens with the concave-mirror projection system. The remarkable result was a device that projected large, upright images. The inverted image formed by the lens, falling a short distance in front of the focal point of the concave mirror, served as an object for the concave mirror, which turned it upright and magnified it. In this way, even a concave mirror of short focal length, within the manufacturing capabilities of the late sixteenth century, could be used to project life-sized images into a darkened room, given an appropriate lens. Della Porta used this device, which doubled as a primitive reflecting telescope, not to paint but to project extravagant theatrical performances for aristocratic audiences seated in his darkened chambers.

Where does this leave the earlier paintings discussed in Hockney's book? At best, the device that Hockney claims was used by artists from the 1430s had perhaps a 35-year working life as an artist's instrument over 100 years later, assuming that it was not just an amusing toy for those unable to draw, as della Porta himself suggests. If Caravaggio used a camera obscura, he would have had every opportunity to avail himself of the latest technology — della Porta's combined convex lens and concave mirror, an instrument that goes unmentioned in Hockney's book. If fifteenth-century painters ever used the camera obscura, however, then they used a simple hole in the wall: no mirror, no lens.

→http://www.stanford.edu/group/shl/Eyes/hockney