It has been disclosed that the Italian Master painter Caravaggio used an early form of photography to project images of his subjects on to a canvas and then painted over them.  It is claimed that the 16th century artist used modern darkroom techniques to create his work, 200 years before photography was invented.  Apparently this explains why most of his subjects were painted as being left-handed because the image was projected reversed.  It was David Hockney  who stated that many old masters used optical instruments to compose their paintings.  It is claimed that Caravaggio used many substances including mercury and fireflies to fix the image.  and that the continued use of mercury could have explained his violent temper.
I have already read that some modern day painters use image projection to set the layout and scale of their pictures, but I am amazed that the whole business started over 400 years ago.

Hockneys demonstration on TV of this was wonderful!
Definitely not cheating, we were all shown the technique at art school!

What's more: some close studies clearly show the use of a technique that's very similar to what we now know as 'Lens Blur' or 'Bokeh' in a number of 15th century paintings (including some of Jan Van Eyck, 1390-1441)...

Yes I  read this in the Telegraph today  I would say that he was just ahead of the game, and some time well before Photography..  I have used pinhole camera's and it is fun even if it is upside down...  The wizards also had ways with paint making that few others were able to emulate..

It is also clear that some of the perfection in perspective could only be possible with these techniques..

For the sake of historical truthfulness, l think I may have to correct my earlier reply: I thought I remembered a lecture on the use of lens blur in the paintings of Jan Van Eyck, but after reflection I think it was in fact Jan (Johannes) Vermeer who used this technique... and that was 1.5 centuries later.
My apologies for my failing memory... ;o)

There is only one form of cheating in art and that is the copying of anothers work and that is called forgery. Everything else is fair game and for years I have realised that many photo realistic paintings are  copies of a photo projected onto the canvas and sketched then painted.