A little over three months ago I wrote discussing artist, David Hockney's, theory that important sixteenth and seventeenth century artists such as Velázquez, Dürer, Vermeer, Chardin, Hals, Canaletto, Caravaggio, even going back perhaps as far as Raphael, used so-called "optical enhancements" to make their paintings more realistic. In other words, they projected images of their figures (mostly portraits) onto their canvases for drawing purposes. At the time, I was a little vague just as to how this feat might have been accomplished in the absence of opaque or slide projectors. Well, thanks to the optics research of Charles Falco of the University of Arizona, appearing in the July (2000) issue of Optics and Photonics News, I can now be a little more specific. Incidentally, thanks to Falco's research, Hockney's theory seems no longer theory but scientifically proven fact.
Most of us know that concave mirrors have the ability to focuses light to such a degree that Archimedes could concentrate sunlight accurately and intensely enough to set fire to the Roman Fleet off the coast of Greece in 212 BCE. But it was news to me, and probably to most artists, that they also have the ability to project images albeit upside down (but not backwards as would a lens). Falco studied the double portrait Husband and Wife by Lorenzo Lotto painted around 1523. Neither the portrait nor Lotto are of any great artistic significance, but to Falco, it was an evidential gold mine. The painting depicts two figures sitting at a table covered by a geometrically patterned cloth. It was drawn by angling the concave mirror slightly away from a straight on frontal configuration thus causing the projected image to also angle off to the opposite side where the easel and canvas were placed. With sufficiently strong sunlight upon the subjects, an accurate, highly detailed, sharply delineated, projected image could be traced onto the canvas.
But one of the problems inherent in the use of any lens (including concave mirrors) is that they have a depth of field. That's the circular area in which focus is acceptably sharp. In Lotto's painting, the pattern of the foreground cloth appears to lose focus, at a point midway towards the rear of the table where it would naturally do so on film, except that in this case the "film" is Lotto's canvas. The human eye would compensate for this problem by automatically refocusing on the rear portion of the design. It would appear Lotto tried to do so too by moving slightly the mirror, which, while improving the focus, coincidentally changed the vanishing point for the pattern. It's not very noticeable to the eye (which would explain why it has gone unnoticed for centuries) but using exacting measurements and mathematics, which are frankly way over my head, the error becomes obvious. And that's how it was done.
However, art historians, while sometimes startled by such revelations, seem more bothered not by the fact that highly respected artists from the past used them, but by the fact that for some period of time at least, they were constricted by them. Falco also studied twelve different head and shoulders portraits of various sizes by a variety of different painters from this time period (1450-1560). By using the space between the pupils of human eyes as a fairly limited constant, then adjusting for varying head angles, he was surprised to learn that every one of them was painted with a magnification of -90%. Given the fact that so many artists would be unlikely to all choose to be so consistent, only some form of optically imposed limitation could account for such a phenomenon. Eventually of course, artists developed methods and techniques to overcome many of the limiting functions of these tracing aids, though not always with perfect success. But it would seem that what they gained in convenience, accuracy, and speed, they may have lost in artistic freedom by having to adjust their compositions to accommodate the peculiarities and limitations of their new-found tracing aids. And this is the most important new factor in our study of art history from this period.