In his book Secret Knowledge the artist David Hockney set out his controversial thesis that various Old Masters used a camera lucida and other optical devices. According to Hockney this can be seen in the shift in style of portraiture in the fifteenth century.
Hockney's research was first made by public in an article by Lawrence Weschler called The Looking Glass in The New Yorker magazine in January 2000. Weschler published a follow-up article Through The Looking Glass in 2001 which contains paintings and drawings Hockney used to prove his theory (all reproduced in Secret Knowledge).
Why All the Fuss?
In part it was the fact that a painter, albeit a distinguished one, was treading in the realm of art historians. In part is was most of Hockney's evidence was circumstantial, that there was a lack of corroborative evidence (though Hockney said that the lack of preliminary sketches by some prominent portrait artists was evidence of their use of optics). And in part it was the belief that an artist should achieve their results by skill alone, not 'cheat' by using optical aids. There has been much debate, without a conclusive answer being reached, and it probably never will be, given the lack of corroborative evidence. If you look at the visual evidence Hockney presents it's clear that optical devices were used, but the question remains: to what extent?
But it doesn't detract from the work of the Old Masters unless you require an artist to achieve results by with any technical assistance. After all, as Hockney says, "The lens can't draw a line, only the hand can do that ... look at someone like Ingres, and it would be absurd to think that such an insight about his method undercuts the sheer marvel of what he achieves." Strange how there haven't been similar objections to the use of perspective rules and grids by artists.