A few months ago I had a discussion about David Hockney’s hypothesis that many classical painters ( such as Ingres or Carravagio) had used a Camera Obscura. I think these were used somewhat, but not as much as Hockney stipulates.Here is Gregg Kreutz’s argument against Hockney’s “proofs”.
http://www.kirkrichards.com/Gregg_Kreutz_article.htm
Stephan Giannini 