The circular spots of light shining on the ground vary in size depending on how high the canopy is above the ground. A high tree canopy leads to larger circles with softer edges. Below, in this early Albert Bierstadt painting called “Sunlight and Shadow” the effects of dappled light are worked out extremely convincingly.
When bundles of light pass through small spaces between the leaves, each of those spaces act like a pinhole camera. The parcels of light are essentially like conical shapes of illumination radiating from each pinhole in the foliage.
The circles of light touching the ground are actually projections of the disk of the sun. In fact on days with a partial eclipse of the sun, the circles of light will appear as half circles. In the 8x10 inch oil study above, the circles of light on the roof of the shed are about a foot in diameter.
When each cone of light intersects a sloping surface like a wall, it results in an elliptical shape. On a vertical surface parallel to the picture plane, the long axis of that ellipse will always angle back toward the source of the light.
In the detail above from Dinotopia: Journey to Chandara, the light is coming from the right. The circles become ellipses as the form of the ship's red hull curves away on the left side.
Therefore, ellipses of light projected on a wall in front of us will slope upward to the right only if the sun is also coming from the right, as it is in this photo also.
Apparently N.C. Wyeth was unaware of this principle when he designed this otherwise fine illustration of Ben Franklin’s arrival in Philadelphia. The result is an error in lighting. According to the cast shadows from Franklin’s leg and from the tree branch at the bottom of the picture, the light is coming from above and to the right. But the ellipses of light on the wall point impossibly to a light source above and to the left.
Further discussion of dappled light, with photographic examples, on Edward Tufte's website, link.
Tomorrow: Business Cards, Illustrated