Hans Holbein's Portrait of Georg Gisze in the Gemaldegalerie, Staatliche Museen, Berlin is adduced by Hockney as evidence that the artist traced an optical projection. The following is based on:
In Secret Knowledge Hockney notes a slight break in one perspective line and attributes it to Holbein secretly building an optical projector, placing the table and carpet in the sun (needed to produce a visible image), tracing the front half of the carpet, then re-focussing and tracing the back half. However, such kinds of slight breaks appear in many many paintings and murals—paintings that we can be positive were not executed by means of tracing optical imges. Moreover, the coin box is indeed slightly skewed compared to what would be geometrically correct but such a deviation would have arisen had the artist moved forward when drawing the box. In fact, if you do a full appropriate perspective correction of the table (even each half separately), according to a putative "exposure," then the shape of the table is very awkward and implausible. Taken together, Hockney's visual evidence here is very weak and unpersuasive.