Hockney and Falco call Lorenzo Lotto's Husband and wife in the Hermitage Museum in St. Petersburg their "Rosetta stone," which "proves" that Lotto created an optical projector and that "it simply is not possible" that Lotto didn't use optics when executing the painting.
The following is based on:
A crucial, but unstated, assumption in the Hockney/Falco argument is that the physical carpet in Lotto's studio was symmetric, at least to the very high level of precision they specify in their optical fits. But is their unstated assumption valid? These carpets, now called "Lotto carpets" after the artist, were hand knotted by pairs of young, poor, uneducated girls in the Ushak region of present day Turkey. (Such carpets were not woven with modern shuttles on a loom.) These girls surely worked at different rates, took their breaks at different times, made separate errors, and so on. Then, the carpets were rolled up, transported hundreds of miles on donkey backs or in carts, unloaded, repacked onto wooden sailing ships and sent on the arduous journey to Venice (or elsewhere), where they were then unpacked, shipped to the local merchant and sold, rolled and transported to the artist's studio, and stored and then unpacked. A modern study of these carpets in museum collections reveals that the vast majority are spatially asymmetric to an extent as large as the deviations (the purported "optical evidence") in Lotto's painting. There seems to be no way—even in principle—for Hockney and Falco to prove that Lotto's particular carpet in his studio was symmetric to the scale of less than a percent or two, as is necessary and merely the first step to justify their optical claim. (They have never given persuasive evidence that Lotto's particular carpet was highly symmetric—how could they? Finding some, or even many, symmetric carpets could not prove that Lotto's particular carpet was symmetric.)
Hockney and Falco claim that the kinks and deviations in the keyhole pattern in the painting arose from Lotto taking his carpet into the direct sunlight (needed to produce a visible image), then projecting the image on the canvas (screen), tracing it, and refocussing twice. Robinson and Stork showed that their setup simply could not work. Specifically, Hockney and Falco failed to include the 116-cm-wide canvas (screen) into their setup, and that when included the light must strike the mirror at a large angle and this, in turn, leads to significant off-axis aberrations. This, in turn, means that the projected image would not have gone in and out of focus, as claimed by Hockney and Falco. In short, the fundamental process underlying their explaination would never have occured.
Hockney and Falco point to the top of the keyhole in the rendered carpet and claim it is "blurry," much like an "out of focus" projected image. I have spent many hours studying this painting, with the Chief Scientific Conservator at its home and the curator of Italian paintings at its home, the Hermitage in St. Petersburg. There is no evidence—none—that there were tracings. Moreover, their explanation that Lotto painted the image "blurry" to reconcile the current sharp image at the center with the memory of the position of lines from the second image is highly unorthodox; they do not explain why this unusual process would have occured reconciling putative exposure 2 and 3 but not when reconciling putative exposure 1 and 2.
Lotto wrote a personal notebook or Libro di spese, bearing all manner of personal notes, travel plans and so on. Hockney and Falco have pointed to following passage in Vincent Ilardi's recent book on optics in the Renaissance as support for their claim about Lotto, but the proper reading of this passage, in context, shows just the opposite:
In 1549 [Lorenzo Lotto] paid the enormous sum of 22 Venetian lire for a ‘big crystal mirror’ [speculum] ordered from Venice to replace a broken one while he was working in Ancona. ... In sum, these few entries in Lotto’s account book and the evidence presented above demonstrate that mirrors were used by most or many artists to pro ject images and/or control the accuracy of their visual observations.
There is no textual evidence in the Libro di Spese that this mirror was concave, as needed for a projector, rather than plane or convex. There would seem to be no passages here that suggests that this artist had discovered what then would have been a complex optical procedure. Ilardi concludes “the [documentary] evidence on the use of concave mirrors to pro ject clear images for pictorial composition seems to be scanty or missing altogether...”
Beyond rebutting Hockney and Falco's claim for this painting—their "Rosetta stone"—the computer ray tracing methods promise to be of value to the analysis of paintings that are more plausibly argued were executed with the help of optical projections, paintings perhaps two centuries later.