The Rokeby Venus by Diego Velázquez is a good example of a very common illusion in many paintings:
Most viewers would say this picture depicts a woman viewing her own reflection in a mirror. But based on the orientation of the mirror, it’s actually physically impossible for her to see her own reflection. Since we can see her face, then if she could see face in the mirror, her head would have to be positioned between us and the the mirror. At best all she would be able to see is us (or rather, the painter painting her picture).
Art critics have suggested that there’s another problem with this particular painting. The head of Venus in the mirror is also much too large, they say: it’s roughly the size of her face, which would be impossible if the observer were standing relatively close to Venus — it should probably be at least half the size of the real thing (though if you assume the viewer is standing a relatively large distance away, this problem disappears).
Marco Bertamini, Richard Latto, and Alice Spooner confirmed this “Venus effect” by asking twelve observers to describe the picture in their own words. Seventy-five percent said it depicted a woman looking at herself in a mirror. So even though the situation they describe would be physically impossible, it’s by far the most common explanation of what they see.
After looking a dozens of famous paintings involving people regarding themselves in mirrors, the researchers found that in most cases the painter made some effort to replicate the experience of looking in the mirror, not necessarily what the painter might have seen while painting. Take a look at these two pictures, for example:
The painting on the left, Vasari’s Toilet of Venus, shows Venus looking at the familiar mirror. The image we see is a face looking back at her. But if this was a real scene, even acknowledging that she couldn’t actually see herself in the mirror at the same time we do, the orientation of the face in the mirror is wrong, too. For us to see a face pointing to the left, the mirror would have to be much closer to use than Venus is. But from the position of the attendant’s arm relative to Venus’, it’s impossible for that to be the case. So the painter couldn’t have seen this view while he was making the painting.
The other painting, Couple with Mirror by Hans von Aachen, offers and even more implausible view. It’s hard to imagine how it would be remotely possible to see the woman’s face based on the orientation of the mirror in the painting. What’s more, though it’s difficult to see in this jpeg, the mirror-image’s eyes are shifted to the left, looking back at the woman, while she stares straight ahead.
The researchers say that artists deliberately manipulated these images to create the impression we get when looking at ourselves in the mirror. If you can see your mirror image, it’s always looking straight back at you, just like the images in the paintings appear to be looking back at their beholders. If the painters had actually painted these images the way they saw them, the effect may have been less realistic.
Bertamini, M., Latto, R., & Spooner, A. (2003). The Venus effect: people’s understanding of mirror reflections in paintings Perception, 32 (5), 593-599 DOI: 10.1068/p3418