Jan van Eyck's pair of portraits of Cardinal Niccolò Albergati—a silverpoint study of 1431 in the Kupferstich Kabinett in Dresden, and a larger, formal oil portrait in the Kunsthistoriches Museum in Vienna—are adduced by Hockney and Falco as evidence this artist traced optically projected images nearly two centuries earlier than scholars previously thought. The following is based on:
In brief, the copy is magnified roughly 1.4 compared to the original. There are two types of evidence that led Hockney and Falco to claim that van Eyck copied the silverpoint study by means of an epidiascope or opaque projector: the fidelity of parts of the copy and a relative offset of parts of the images, such as the ear. They believe van Eyck built an optical projector, projected the silverpoint study onto the oak panel support, traced portions, then "made a mistake" and shifted the projection (twice) and traced other portions.
Several years after Hockney and Falco made their claim that van Eyck used an epidiascope or opaque projector when copying the Albergati portrait, Thomas Ketelsen and his team—which included two physicists—discovered pinprick holes in the silverpoint on contours. This is the first truly independent evidence in the debate over the Hockney theory—that is, evidence that could not have been used to craft an explanation. This evidence is completely compatible with the Ketelsen team's mechanical explanation, and cannot be explained as part of the optical explanation. Indeed Ketelsen and his team of experts concluded van Eyck used a reducing compass or compasso da reduzione or Reductionzirkel when making the copy. Such devices were known from Roman times and found in Pompeii, and appear in works by Leonardo and other Renaissance artists; the Museum of Science in Florence has many surviving examples. Falco counters of the number of pinprick holes: "These are far too few to have resulted from someone having used a compass or proportional divider to transfer the sub-mm fidelity of the entire surface of the drawing to the painting." There seems to be no way, even in principle, to justify such a statement and Falco gives no evidence that could justify that statement. Moreover the fact that modern realist artists drawing "by eye" (no prinprick holes) can achieve high fidelity argues against the optical claim. More to the point, though, an artist using a reducing compass needn't leave pinprick holes (he need merely gently touch the original to mark the separation of key features), so nine holes found by Ketelsen and his team is the lower limit to the number of touches of the reducing compass; the artist could easily have used the device a dozen or even a hundred times. In fact, art historians have published several studies where they infer the use of a reducing compass without pinprick marks whatsoever. Hockney and Falco give no evidence and provide no persuasive explanation for the existence of these distinctive pinprick holes, and state merely that there's "no way to know" how they arise.
Several modern realist artists copied the silverpoint "by eye," by reducing compass, and by grid construction, and matched or surpassed the fidelity in van Eyck's copy, thereby showing that van Eyck did not need to use optics. (Of course, there is no documentary evidence van Eyck had even seen an optically projected image, or traced one for any work, including the Albergati portrait while there is ample evidence—renderings and documentation—that artists as early as Roman times used the simple mechanical device.) Note too that the motivated van Eyck would surely copy his work better than these arists, since it was his own study drawing. Finally, the fact that the copied face is wider than the original silverpoint (and the shift in the ear) are easily explained by the artist merely drawing the ear where he wanted.
In sum, a large number of experts have thoroughly examined the optical projection claim for this work and rejected it. Especially noteworthy are the comments of Hockney's long-time friend, contributor to Secret Knowledge, and author of The science of art, Martin Kemp of Oxford University: "My own view is that Campin and van Eyck may well have been inspired by optically generated images—the camera obscura was well known to mediaeval natural philosophers—but probably did not actually use them directly at any stage in the making of their pictures." (emphasis added)