Leonardo knew more about the workings of nature than anyone who had lived before him and that the eye does not see everything with crystal clarity at every glance.
The Mona Lisa can look different each time we look at her. Our eyes fill in the vagueness of her face and the image is harmonised by the skill of an artist and scientist who has fully utilised his knowledge to produce a masterpiece of Western Art.
That enigmatic smile! (detail) (s)
This detail highlights the subtle nature of Leonardo's sfumato effects. It does not explain the fascination with the Mona Lisa's smile, or the speculation and theories surrounding the painting.
The Mona Lisa is thought to be Lisa Gherardini, the wife of the Florentine silk merchant Francesco del Giocondo. According to Vasari, Leonardo employed singers and bell ringers and other entertainers to keep Lisa amused while he was painting her, hence the smile. Others have claimed that the smile evolved during the years that Leonardo reworked the painting.
We will probably never really know the truth about any of these claims and in a way it does not matter, we need only to study the details of the painting to be assured of Leonardo's genius.