Go to the museum and notice that most of the paintings look very different from photography. Granted some are very realistic looking, particularly the French academics of the second half of the 19th century. But even those have a "drawn" look that is non photographic.
A British painter, David Hockney has written a book entitled "Secret Knowledge" that attempts to explain away the drawing abilities of the great masters of painting as being actually the products of camera obscuras. A camera obscura is a lens and box machine known from the renaissance that will project an image onto paper allowing it to be copied. It is essentially a camera without film.This is a case of wishful thinking on the part of a man who cannot draw well himself who would like to prove that no one else could draw either. But this is part of a larger mistaken idea as well. That is the idea of painting as a window.
If you look at the painting above it would seem obvious that it is not a traced drawing from a projected image.
Here is is an Ingres (image;artrenewal.org) that is a good example to explain with. At a glance it looks photographic, but upon closer inspection what it is, is convincing. In fact the more you study it the more abstract and otherworldly it becomes.
None of the great historic paintings in the museum could be mistaken for vision. Hence
A PAINTING SHOULD NOT BE MISTAKEN FOR A WINDOW. A VIEW OUT OF A WINDOW IS STYLELESS. VISION IS STYLELESS.
Vermeer probably had and used one and there is evidence to support that claim. However there is simply no way a camera obscura explains away Rubens, William Blake or Michelangelo.
The "modern art spokesmen would like us to believe that the artist was merely a second rate camera and made unnecessary by the development of photography. They want us to think that our historic traditional painting is only about its subject matter. They believe that what they call "modern" art is the result of the artist liberated from the demands of representation and free to carry out the"real" business of art
An artists must be more than a meat camera, he must filter vision through his own perceptions and intentions and produce a thing that is more than transcription.
To do this the artist applies to his painting a "treatment". That may include artful coloring, use of form, design, narrative and other schemes to elevate his creations from journalistic to poetic. All of these things involve decision making. If you aren't thinking you aren't making art.