British painter David Hockney, well known for his cool and lovely paintings of California pools, has taken on the new role of detective. For two years Hockney seriously investigated the painting techniques of the old masters, and like any admirable sleuth, compiled substantial evidence to support his revolutionary theory. Secret Knowledge is the fruit of this labor, an exhaustive treatise in pictures revealing clues that some of the world’s most famous painters, Ingres, Velázquez, Caravaggio (just to mention a few) utilized optics and lenses in creating their masterpieces. Hockney’s fascination with the subject is contagious, and the book feels almost like a game with each analysis a “How’d they do that?” instead of a whodunit. While some may find the technical revelation a disappointment in terms of the idea of genius, Hockney is quick to point out that the use of optics does not diminish the immensity of artistic achievement. He reminds the reader that a tool is just a tool, and it is still the artist’s hand and creative vision that produce a work of art.