The painting technique that creates the Mona Lisa’s soft shading is called sfumato.
The Italian word sfumato means “smoky” or “vanished like smoke.” Leonardo used extremely subtle transitions between tones instead of hard outlines, allowing the face, eyes, mouth, and distant landscape to emerge with an unusually atmospheric softness.
The effect is especially important around the corners of the Mona Lisa’s eyes and mouth. Small changes in light and shadow make her expression seem to shift depending on where and how directly the viewer looks. This is why the smile can appear more noticeable in peripheral vision than under close, direct scrutiny.
Sfumato is not the same as chiaroscuro. Chiaroscuro creates drama through strong contrasts between light and dark, whereas sfumato softens the boundary between them. Leonardo also used the method in works such as The Virgin of the Rocks and Saint John the Baptist, helping make it a signature of his painting.