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.
"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."
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.