David Hockney demonstrates his thoughts and findings in studying the grand masters of painting from early Rennaissance till the modern invention of photography in the middle of the nineteenth century. To Hockney it is obvious that the abrupt, seemingly miraculous development in the painters' ability to pin down the liveliness of the human face over a very short period during the 15th century, owes in large part to the use of lenses and mirrors. This secret knowledge of the painterly craftsmanship has been the object of his deep fascination through some years now, and he has also tested some of the techniques such as the so-called camera lucida in a series of portraits of friends and professional relations.