About 20 years ago, David Hockney made in his book "Secret Knowledge" a powerful case for the hypothesis that a revolution was set off in Western painting by the use of camera obscura and other optical instruments from the 15th century onwards.
According to his very convincing thesis, the breathtaking realism of (especially portrait) paintings by Van Eyck, Vermeer, Velazquez etc. could never have been achieved by simply "eyeballing" the subject. By the beginning of the 19th century, the technique was so widespread and refined, that many "paintings" were de facto "photographs" before the advent of photography itself.
Of course, this crucial introduction and generalization of "photographic" painting, without which the evolution of Western art over the past 500 years would be impossible to understand, could be seen as "cheating". Is the controversy caused by "Secret Knowledge" justified? Have art historians given Hockney's thesis the attention it deserves?
Wikipedia on contemporary photorealism: "Photorealist painting cannot exist without the photograph. In Photorealism, change and movement must be frozen in time which must then be accurately represented by the artist. Photorealists gather their imagery and information with the camera and photograph. Once the photograph is developed (usually onto a photographic slide) the artist will systematically transfer the image from the photographic slide onto canvases. Usually this is done either by projecting the slide onto the canvas or by using traditional grid techniques. The resulting images are often direct copies of the original photograph but are usually larger than the original photograph or slide. This results in the photorealist style being tight and precise, often with an emphasis on imagery that requires a high level of technical prowess and virtuosity to simulate, such as reflections in specular surfaces and the geometric rigor of man-made environs."