I personally think that this is just a theory and that David Hockney did prove his theory to the best of his ability for what he had to work with but I think that this theory is sad it portrays art in a way that the artists almost cheated by using these techniques and tools that Hockney shows through out the video.
David Hockneys' theories were well supported because he put up pictures from each century and noticed a big change in 1420's because paintings looked very real with lots of detail and paintings had a lot of light and started to look like photographs in 1600s. David Hockney noticed people in paintings started being depicted left handed instead of right handed which is more common then finding people who are left handed. Then they came up with a flat mirror to reflect off a lens to sketch the image properly so the painting was not backwards.
I was disappointed with his explanation because he's saying artist’s eyes could not paint great detail. Hockney's saying it is impossible for artists to paint without a photographed image which back then was using a mirror or lens to reflect the image back on the canvas and he says that science and math prove it, because it would be extremely difficult to paint for example a chandelier or fabric.
I also was disappointed when he used the glass lens to reflect back what the artist was painting. I agree with what David Hockney's theory is to a point, I was just disappointed that the human eye couldn't do it with out help of the mirror and or lens in a dark room to trace the image and then paint in the detail of a chandelier and fabric. Hockney’s also stated that it was impossible for the artists not to use these lenses and or mirrors because the paintings that he has researched were out of focus which the human eye does not go out of focus.