The optical illusion hidden in the 'Mona Lisa' explained

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The Optical Illusion Hidden In The 'Mona Lisa,' Explained

Art historians say Leonardo da Vinci hid an optical illusion in the Mona Lisa's face: she doesn't always appear to be smiling.

There's question as to whether it was intentional, but new research into a second painting attributed to da Vinci suggests yes, it was.

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"Portrait of a Young Fiancée," which is thought to be of an Italian girl named Bianca, shows the same almost-smile depending on viewing distance and sharpness of the image.

"When she is viewed from close-up the mouth appears to have a downwards slant, making her look melancholic, unhappy and hostile, but when viewed from further away, her mouth appears to take an upward, smile-like appearance making her appear happy and cheerful," researchers wrote.

Click through for more images of the iconic painting:
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The Mona Lisa and the Louvre
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The optical illusion hidden in the 'Mona Lisa' explained
PARIS, FRANCE - FEBRUARY 28: Visitors take pictures of Leonardo da Vinci 'Mona Lisa' inside the Louvre museum on February 28, 2014 in Paris, France. The museum is housed in the Louvre Palace, one of the world's largest museums which opened 1793. (Photo by Christian Marquardt/Getty Images)
Japanese Prime Minister Shinzo Abe (Front) and his wife Akie (3rd R) look at 'La Joconde', a 1503-1506 oil on wood portrait of Mona Lisa by Leonardo Da Vinci, at the Louvre Museum in Paris on May 4, 2014. Japanese Prime Minister Shinzo Abe arrived in France Sunday on the latest leg of a six-nation European tour for trade and security talks at a time of mounting tensions with China. Abe arrived in the French capital on Sunday afternoon and immediately left for a private visit to the Louvre that included stops at the museum's best known works, including the Mona Lisa and Venus de Milo. AFP PHOTO / STEPHANE DE SAKUTIN (Photo credit should read STEPHANE DE SAKUTIN/AFP/Getty Images)
The Louvre is one of the world's largest museums and a historic monument. A central landmark of Paris, where famous sculptures and paintings like Mona Lisa are displayed. (Photo by Jayakumar Radhakrishnan via Getty Images)
Alessandro Vezzosi, the director of the Leonardo da Vinci Museum in Vinci, presents a painting unveiled by the Mona Lisa Foundation on September 27, 2012 in Geneva. The Swiss foundation claims the painting is an earlier version of the 'Mona Lisa' painted by Leonardo da Vinci, although some experts said the claim was unlikely. The Zurich-based Mona Lisa Foundation presented what it described as a 'stunning portrait of Lisa del Giocondo,' along with results from 35 years of research and scientific tests which it claimed proved it was the work of da Vinci and had been completed about 10 years before its famous sister in the Louvre. AFP PHOTO / FABRICE COFFRINI (Photo credit should read FABRICE COFFRINI/AFP/GettyImages)
Picture of a TV screen showing scientific tests made on 'Isleworth Mona Lisa' (L) and the Louvre Museum da Vinci's masterpiece taken during the unveiling by the Mona Lisa Foundation on September 27, 2012 in Geneva of what the foundation claims is an earlier version of the 'Mona Lisa' painted by Leonardo da Vinci, although some experts said the claim was unlikely. The Zurich-based Mona Lisa Foundation presented what it described as a 'stunning portrait of Lisa del Giocondo,' along with results from 35 years of research and scientific tests which it claimed proved it was the work of da Vinci and had been completed about 10 years before its famous sister in the Louvre. AFP PHOTO / FABRICE COFFRINI (Photo credit should read FABRICE COFFRINI/AFP/GettyImages)
Paris, France - February 19, 2014: Beautiful view of the Louvre museum in Paris, France, on February 19, 2014
Paris, France - June 18, 2014: Louvre museum at dusk on June 18, 2014 in Paris. This is one of the most popular tourist destinations in France displayed over 60,000 square meters of exhibition space.
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The researchers also found the smile — or frown — appears to change depending on where it is in a viewer's peripheral vision.

They simulated this effect on test subjects using blur. The blurrier Bianca was, the happier she appeared to be.

The technique in this portrait and in the "Mona Lisa" is called "sfumato," in which da Vinci blended colors and shades to get gradual transitions between different shapes in each painting.

The effect could be hard to pin down because it's so hard to do: the researchers suggest nobody has been able to pull it off as successfully as da Vinci did.

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