The Vignette Composition
A vignette composition is one that has no linear connection to the edges of the picture; the image seems to float in the middle of the paper or canvas. The vignette composition is the weakest of all the types, and is used primarily when a feeling of softness and gentleness is desired; the borders of an image become soft-edged and fade into the background, as in greeting cards. Vignetting may also be used to separate a subject from its environment to increase its importance, as in a portrait; in many of Rembrandt's portraits, for example, the face emerges dramatically from a dark background.
Thus, in order of importance, the four main elements of composition are line, value, color, and texture. If these aspects are well thought out, first individually and then as a unit, effective compositions should result. You can influence a viewer's response to your work by using these tools expertly and with a little personal creativity.
A vignette composition is one whose linear and value elements have no connection to the edges. It is used when gentleness and softness are desired. It can also be used to isolate an important subject, as in a high contrast portrait in which a face emerges from darkness.