In his book The Great Art of Light and Shadow, Jesuit priest Athanasius Kircher explained how contemporary artists used lenses and mirrors to project images onto canvases to speed up the compositional phase of painting. Fortunately for him it was 1646, so there were few art historians to accuse him of blasphemy. Unfortunately for modern artist David Hockney and physicist Charles Falco, their 2001 book Secret Knowledge proposes this same idea, and there are a plethora of art historians ready to excoriate them.
Professor Falco explains the science behind his and Hockney's theory by referring to the "vanishing point" in the Lorenzo Lotto painting Husband and Wife, noticeable as a blurred section of a table. If Lotto was using a lens, the large scale of the painting (almost life-size) would require him to refocus the lens to avoid distortion of the projected image as his artistic focus changed from background to foreground. This refocusing caused the degree of magnification to decrease, and "it didn't just decrease, it decreased by an amount we can calculate," Falco notes. Falco can calculate these differences to an accuracy "of 0.2 percent. The magnification decreases by 14 percent, and I calculate it to an accuracy of 0.2 percent; [Lotto] then refocuses and the magnification changes by a different amount, and then I calculate the change in magnification to 0.2 percent."
Through slides and examples of lens projection, Falco demonstrates these quantifiable errors in the works of the Great Masters not to belittle them, but to explain the methods they may have used to create them. In his lectures, Falco displays a lens-projected outline of a Jean-Baptiste-Simone Chardin painting to challenge the audience. "I've created this composition, the outlines, it's all done for you," says Falco. "You fill in the colors. You can't do it." The lenses, if used, were tools to aid the artist, not a shortcut to talent or genius.