The painter, inventor and polymath Leonardo da Vinci may have had a squint that helped him capture the Mona Lisa’s smile and create his masterpieces, according to researchers.

Doctors from the Optometry and Vision Sciences School at City University of London have diagnosed the Renaissance master with strabismus after analysing the eye alignment of works believed to have been modelled, in part, on himself.

Strabismus is a common vision disorder where one of the eyes looks inward, outward, up or down when the other eyes is focused on an object.

The condition is thought to be beneficial to painters as it lets them better focus on close-up flat surfaces and has been identified in a number of famous artists, including Rembrandt.

There are notoriously few validated images of da Vinci and historians can only speculate on the extent he put himself into his work. However, Leonardo's own text the Codex Atlanticus says that artists can't help but be self inspired, stating: “[The soul] guides the painter’s arm and makes him reproduce himself, since it appears to the soul that this is the best way to represent a human being."

"A number of famous artists have been identified as having strabismus on the basis of the eye alignment evident from their self-portraits, including Rembrandt Harmenszoon van Rijn, Albrecht Dürer, Giovanni Francesco Barbieri (who was also known as Il Guercino, Italian for ‘the squinter’), Edgar Degas, and Pablo Picasso," the study’s author, neuroscientist Professor Christopher Tyler, said.

Analysis of the oil portrait ‘Young John the Baptist’ by Leonardo da Vinci (1513-16, Louvre, Paris) (Christopher Tyler/Creative Commons)

“Some forms of strabismus are thought to facilitate artistic work by suppressing the deviating eye, which creates two-dimensional monocular vision advantageous to painting and drawing”

While having two eyes focused on an object (stereoscopic vision) is integral to our depth perception, it can make translating three-dimensional scenes to canvas more tricky, as each eye will be seeing objects slightly to the right or left.

This can be illustrated by holding a finger in front of your nose and shutting your eyes alternately causing the digit to jump side to side.

If one eye deviates, the brain will deprioritise signals from the unfocused eye, preventing this effect and giving a more static image to work from.

For the study published in JAMA Ophthalmology, Professor Tyler measured the alignment in six art works believed to be partly self-inspired by fitting circles to the irises of each eye and measuring the average deviation from alignment.

A bronze sculpture of David (1473/75) by Andrea del Verrocchio believed to be modeled on a young Leonardo da Vinci (Christopher Tyler / Creative Commons / JAMA)

Works included two drawings, two oil paintings and two sculptures. Among them his world famous Vitruvian Man sketch, the bronze sculpture David – reputedly a depiction of the young Leonardo– and Salvator Mundi, his painting of Jesus Christ which recently became the most expensive in history.

In all cases, the eye misalignment was measurable, though not severe and averaged a −10.3° deviation from the focussed eye across the six pieces.

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This negative number means the eye would tend to look outwards (exotropia) and Professor Tyler argues that da Vanci’s strabismus may have been non-existent when focusing intently on an object, but would come in when he relaxed into painting. This would give him the best of both worlds.

The professor said: “The weight of converging evidence suggests that Leonardo had intermittent exotropia, with a resulting ability to switch to monocular vision.