|Hockney's been floating the theory for a few years now that the camera obscura was in wider use by painters than previously suspected. Without reading the book, I'd say his painter's eye could be trusted to detect machine influence as well as or better than any art historian's. That said, so what? The camera (or camera obscura) is a useful shortcut for drawing, but ultimately it's the sensitivity and/or eccentricity of the painter's eye and hand that makes a "masterpiece." In fact, it's the little subjective distortions--as opposed to perfect point-for-point correlations between depiction and depicted--that make even the most "accurate" paintings interesting.
Which sounds unabashedly romantic, but even in a major "realist" work like David's Consecration of Napoleon I, it's the subjective decisions--the placement of figures, the rendering of surfaces, lighting--which work together to create an overwhelming, convincing fiction of Imperial Power. A painting (or even a photo) can be breathtakingly accurate but still be dead, as far as its effect on us. There are plenty of examples of French and Russian academic painting that demonstrate this. Anyway, all this is to say that the means of visual representation is ultimately less important than the ends.