In late 90’s David Hockney and Charles Falco developed an interesting theory which states that advances in realism in the history of Western art since the Renaissance were not results of the development of artistic technique and skills but primarily the result of optical aids such as the camera obscura, camera lucida, and curved mirrors. In 2001 Hockney published the book “Secret Knowledge: Rediscovering the lost techniques of the Old Masters” which is a extended form of his argument.
I had a opportunity to read the book while I was finishing my postgraduate studies in Barcelona, and I found it very impressive and for me it was certainly an “eye opener”. Here you can find and interview with David Hockney in 3 parts where he speaks about the book and his findings.
I hope it will impress you enough to go and find the book, since by my opinion his work is a “must read” for every realist painter.