The painting is one of the starkest and most austere of Zurbarán's representations of saints. The face is largely obscured by the cowl and its shadows and the light is concentrated on the coarse patched habit and on the skull clasped to the saint's body.
It is probably of the early or mid-1630s, preceding a second picture of Saint Francis in the National Gallery which is signed and dated 1639. In that picture the figure, wearing the same habit, is placed in a landscape and directs attention outside the painting, holding a skull on a rocky ledge. Meditation on death was favoured, especially by the Jesuits, as a religious exercise, and saints contemplating skulls are frequent in Spanish and Italian painting in the early 17th century.
From The National Gallery Podcast: Episode Seventeen, March 2008