Enigmatic smile's secret revealed

The Mona Lisa hangs in the Louvre Museum in Paris. Photo / Supplied

British researchers believe they have unlocked the mystery of the Mona Lisa's enigmatic smile - by analysing another, recently discovered masterpiece by Leonardo da Vinci.

By looking at La Bella Principessa, the portrait of a Milanese nobleman's daughter, researchers found intriguing clues as to how the Renaissance artist painted the Mona Lisa in such a way that her coy smile appears most pronounced when viewed from an angle and less so when looked at directly.

The researchers, from Sheffield Hallam University, believe the same effect was created by a painting technique known as "sfumato", meaning soft or pale in Italian, in which subtle colours and shades around the mouths of the subjects create a clever optical illusion.

If one focuses on the eyes of the subject, the lips appear to slant delicately upwards in a tentative smile, but if one looks at the mouth directly, they appear flatter.

In both paintings, Leonardo exploited differences between our peripheral vision and direct sight.

The researchers have described Mona Lisa's smile as "uncatchable" because it seems to disappear as soon as the viewer looks at it directly.

The researchers, Michelle Newberry and Alessandro Soranzo, organised several experiments in which viewers were asked to look at the Mona Lisa and La Bella Principessa from different distances and angles.

They were also shown digital copies of the paintings which had been blurred to different degrees. The blurring mimicked the effects of peripheral vision, in which objects are seen less distinctly. The more blurred the images were, the more Leonardo's subjects appeared 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 published by the journal Vision Research. "It was found that a perceived change in the slant of La Bella Principessa's mouth influences her expression of contentment."

The viewers said both the Mona Lisa and La Bella Principessa appeared more "smiley" when viewed from a distance or when blurred.

The scientists also showed participants images of the two women with their mouths or eyes blacked out. With the mouths obscured, the viewers reported they could discern no change of expression, suggesting that the key lay with how the lips were painted.