mona lisa

Why is it that Mona Lisa looks like she’s smiling more from an angle than if you’re looking at her head-on? Why does it disappear as soon as you look at it, and how in the world was Leonardo da Vinci able to paint such an amazing optical illusion some 500 years ago? That massive secret — the “uncatchable smile” — that researchers and art aficionados alike have been scratching their heads over for centuries may have finally been figured out.

In an essay published in Vision Research, researchers from Sheffield Hallam University used a recently discovered painting by da Vinci — La Bella Principessa, the portrait of a Milanese nobleman’s daughter that has the same tricky, coy smile that seems to change depending on the angle from which you’re looking at it. Da Vinci painted the portrait, which translated in English is entitled Portrait of a Young Fiance, years before he started on the Mona Lisa. The painting was originally thought to be done by an early 19th-century German artist. The researchers believe that both portraits were painted with a technique known as sfumato, which is Italian for soft or pale and incorporates subtle shading around the mouth, creating an optical illusion and highlighting the differences between our peripheral vision and direct sight.

The researchers, Michelle Newberry and Alessandro Soranzo, conducted a series of experiments asking viewers to look at both the Mona Lisa and La Bella Principessa from different distances and angles. They were also shown copies of the portraits that were blurred to different degrees, intending to imitate the effects of peripheral vision — and the more blurred they were, the more the subjects within the paintings seemed to smile.

“La Bella Principessa’s mouth appears to change slant depending on both the viewing distance and the level of blur applied to a digital version of the portrait,” they wrote in a paper in Vision Research. “It was found that a perceived change in the slant of La Bella Principessa’s mouth influences her expression of contentment.”

The researchers also conducted the experiment again, but with the women’s eyes or mouths blacked out (which sounds pretty horrifying, TBH). Viewers saw no difference in the degree of smile, which made it clear to researchers that the entire illusion is within the lips and mouth.

“. . . the existence of a similar illusion in a portrait painted prior to the Mona Lisa becomes more interesting,” the researchers wrote. “The question remains whether Leonardo da Vinci intended this illusion. In any case, it can be argued that the ambiguity created adds to the portrait’s allure.”

The researchers told Discover that, although many have tried, no one within knowledge has been able to quite master the “uncatchable smile” technique in their work quite like the great da Vinci.

“Given da Vinci’s mastery of the technique, and its subsequent use in the Mona Lisa, it is quite conceivable that the ambiguity of the effect was intentional,” Soranzo said. “Many of Leonardo’s followers used a similar technique, but without they haven’t been able to achieve the same result.”

The next goal for the researchers? Looking at da Vinci’s work next to his followers’ to see if they can crack that subtle smile for good. Oh, Leonardo, you brilliant mastermind — keeping us on our toes for half a millennium.

(Images via Twitter.)