They are a power when the idea of bulk and reality are part of his subject matter. They are a limitation when he wishes to be understood in general, subjective terms. It is often an advantage to a portrait painter to be able to create the illusion of reality upon his canvas. It is often a disadvantage to a painter of the Crucifixion to have his work look like a tableau vivant. It is fatal when his aim is to express some general aspect of the spiritual significance of his subject.
Tempera lends itself readily to any system of light and shade, abstract or scientific.
But its limited value range, overbalanced toward light, makes it less persuasive in a realistic scheme than oil painting. Any system of light and shade may be employed in tempera, from the elementary, ideographic, medieval types, to the most highly descriptive analysis of form. A great factor in naturalistic painting, in which oil painting outstrips tempera, is the recognition of modifications of local color according to the degree of illumination. In nature, a change of value is always accompanied by some change of color and intensity. A change in value in painting is also accompanied by some change of color and intensity, but not necessarily the same change which takes place in nature.