Have you ever noticed that even though some artists have painted areas of light and shadow in their paintings, their pictures still seem to have no unifying sense of light? That is because they have incorrectly painted the planes that make up the objects in the painting.
Planes, as they pertain to painting, are one of the most essential concepts for creating a sense of light and space in your work. Planes help create the illusion of form. It is the ability to correctly identify where the planes are on a form and their angle to the light that helps to make a successful painter. Whenever you see a plane change, you must also change the hue, temperature or value to record it. Imagine the facets of a diamond. The flattened areas are planes. By observing the way light changes on these planes, you can create a believable form.
When painting the landscape, the idea of planes still applies. Think of the earth as a large, horizontal plane. Trees and buildings would be upright planes and hills and mountains would be inclined planes. We know that light from the sun falls in parallel rays. When this light falls on different objects in the landscape, it is the direction of the light in relation to the angle of the plane of the object that determines the brightness. When the plane of an object is perpendicular to the direction of the light---that place is the object’s brightest point. It is the consistency with which you paint this relationship that creates a unifying sense of light in your work.
The outdoor painter has the added challenge of atmospheric recession and the suns movement across the sky. As the sun moves, the angle of the light changes…and changes the way it interacts with the scene. This is why it is imperative to lock in the essential divisions of light and shadow as quickly as possible when painting from life.