David Hockney's Secret Knowledge (part 1/8)
"In the 2001 television programme and book, Secret Knowledge, Hockney posited that the Old Masters used camera obscura (and lucida) techniques, utilized with a concave mirror, which allowed the image of the subject to be projected onto the surface of the painting. Hockney argues that this technique migrated gradually to Italy and most of Europe, and is the reason for the photographic style of painting we see in the Renaissance and later periods of art."
So, after you've watched all 8 parts, check out this lengthy video of Charlie Falco's presentation. Very compelling.