Renaissance Puzzle

Scientist says painter David Hockney was wrong about optics.

You may have heard the theory put forward by painter David Hockney that Renaissance artists used optical devices, such as a camera obscura, to project a scene or subject onto canvas, creating a perfect image from which to paint. They could achieve realism by tracing these projected images.

However, many art historians and physicists disagree. It was only in the early 18th century that artists are generally acknowledged to have used such projectors, like Canaletto, famous for his paintings of Venice. It seems hard to believe that artists could have been using sophisticated optics centuries earlier, from around 1420. "The issues I raised have disturbed some people," said Hockney.

Now, a Stanford University researcher has used computer analysis to show that one 17th century artist did not in fact use such optics. Physicist and art historian David Stork, a long-standing critic of Hockney's theory, believes his findings, presented at the Electronic Imaging Conference in San Jose, California, undermine many aspects of it.

According to New Scientist, Stork used computer imaging software to analyse the shadows in Georges de la Tour's 1645 painting Christ in the Carpenter's Studio in a bid to plot the direction and intensity of the light illuminating the scene. This allowed him to determine whether the candle in Christ's hand was the only source of light. To illuminate the scene brightly enough to project it onto the canvvas, de la Tour would have needed an external light source, probably the sun.

Stork claims his analysis shows that a candle was indeed the only light source in the scene. He also says that given the type of lenses or concave mirrors available at the time, the brightness in the scene would have been reduced around 1000-fold at the canvas, making any projected image all but impossible to see and trace, unless several dozen oil lamps or hundreds of candles lit the scene. As well as showing that the shadows cast can be plotted back to the candle, Stork's software indicates that the way light rays are reflected off Joseph's head are consistent with the candle being de la Tour's only light source.

Yet Charles Falco, a physicist at the University of Arizona in Tucson who worked with Hockney on his theory, is unconvinced. "Artists would not have felt compelled to trace shadows as they were cast, but instead would have made the shadows consistent with the overall scene." Stork, however, believes this is unlikely because the artist would have had to project the night-time scene during the day and then completely rework the painting at night to make the light look reallitic.

In March, Thomas Ketelsen, a curator at the Museum of Prints, Drawings and Manuscripts in Dresden, Germany, will publish separate evidence that also seems to show Hockney was wrong. The similarity between Jan van Eyck's drawing Portrait of Niccolò Albergati and a larger oil painting of the same name could only have been achieved using optical projections, Hockney has argued. Yet Ketelsen has, with a microscope, found evidence of previously unseen pinpricks in the drawing, suggesting the copying method was mechanical, not optical. He suggests the artist might have used a "reductionzirkel", a type of reducing compass.

The pinpricks, though, could have been made 50 or 500 years after van Eyck's death by someone wishing to copy it; "Holes can't be carbon dated," Falco points out. Stork, however, thinks the mounting evidence can't be ignored; "The evidence doesn't support Hockney." According to Hockey, "The debate is fascinating, but it cannot end just because someone found pinpricks."

This story appears in the 15th January 2005 issue of New Scientist.