First, I must say that I am not convinced there is any validity to David Hockney's theories, regarding the use of drawing aids such as special mirrors, the camera obscura, and camera lucida by renaissance masters. At least, not to the extent that he acredits the use of these tools to the works of master artists. While there may be some truth to his claims/theories, I don't feel that he presented a strong case.
In his film, Hockney makes many fallacious arguments which I will address. One example of such fallacy is in his implied claim that because he is unable to draw hard-to-draw objects, such as Van Eycks's chandelier, no one else could be capable of it. He then proceeds to build a large portion of his theory using his own inadequacies as a base for this. I find this to be absurd beyond words.
I almost laughed out loud in class when I saw this clown "demonstrating" how artists would use the camera obscura to trace objects... It certainly didn't appear that he was having a whole lot of success with this method. It's no wonder he didn't bother to use this device to actually complete a painting or, for that matter, a complete drawing. And his demonstration with the camera lucida was embarrassing. I, personally, can produce a better drawing than that, completely freehanded (and I am, by no means, a master). So what exactly did he demonstrate? The answer, in my opinion, is absolutely nothing other than his desperate need to somehow discredit the work of masters who could fit everybit of Hockney's talent onto one of their eyelashes. Many such masters exist today, by the way. And, ok, these devices can be used to roughly trace an object. But it appears to be more trouble than it's worth for someone who actually has a highly trained eye and plenty of experience in drawing from life...like a master artist, for example.
While there were several other fallacies in Hockney's argument, I will address only one more, one that I found particularly humorous. He refers to left handed subjects in paintings which would seem to suggest that the artists were working from reversed images. Here is the problem with that: First of all, he only showed one (maybe two?) example(s) of this. The example that I recall involved two human figures and a monkey. He notes that even the monkey is left-handed as if this were to further his evidence. So we are to believe that a monkey held a pose, while an artist traced this by means of a projection device? Really? Regardless, there are other possible explanations for why left-handedness may have appeared in the work we did actually see. Maybe the artist was left-handed and preferred to portray his subjects as such. Maybe the subjects actually were left-handed, as unfathomable as that may seem. And maybe, just maybe, one doesn't have to be left-handed in order to hold something in one's left hand.
Assuming Hockney is correct, what are the implications of this for modern day artists and art historians? The implications are very insignificant, in my opinion. A master artist does not and cannot rely on mere tricks and gadgetry to somehow magically produce a work of fine, hand painted art. As even Hockney noted, these paintings still had to be painted. And I know from experience that drawing is the easy stage in a painting if you're aiming for "true to life" realism. For the works of masters like Van Eyck all the way to Bouguereau and Sir Lawrence Alma Tadema, vast knowledge of materials and technique was required. A very sensitive eye, something that cannot be overstated, was/is required to discern relationships between values and subtleties in color. And a masterly glazed oil painting requires planning and forsight that can only come with great experience, knowledge, and care. But let's not forget the most important aspect of all -- Composition. A camera obscura can't compose an image so that it leads and holds the eye with intention. A camera obscura doesn't make value and color choices. If artists ever used one, it would have simply been to trace an outline of something. How would this negate the fact that they took the painting on from there? I feel that it would be a terrible shame for anyone to conclude that master artists have been discredited or, in any way, devalued even if it was proven that they used such projection devices. I would challenge anyone in the entire state of Arkansas to use modern, superior, projection equipment and create a painting that rivals the best examples of a Van Eyck or a Bouguereau...I would certainly like to see Mr. Hockney give it a try.