A box is not round. It is made of flat sides called planes, and each plane faces one direction. So each plane catches one amount of light.
Key idea
Give each flat plane one even value. The plane facing the light is lightest, the plane turned away is darkest, and the values change sharply at the edges.
Why it works
Every point on a flat plane faces the same direction, so every point receives about the same light. At an edge, the direction changes all at once, so the value changes all at once too. A round form turns gradually, which is why only round forms get soft gradations.
Worked example: Three planes
A box sits on a table, lit from the upper right. You can see its top, its left face and its right face. Which plane is darkest?
- The light is above and to the right.
- The top faces up, toward the light, and the right face faces it too.
- The left face is turned away from the light.
Answer: The left face.
Common mistake
People blend tone across the edges of a box as if it were round, and the box turns soft and lumpy. Keep each plane one value, with crisp changes at the edges. A plane may darken a little toward its far end, but keep the jump at every edge.