The simplest form of compositional structure is symmetry whereby both sides of the image are identical, but opposite, around a vertical central axis. The three main elements of composition are structure, line dynamics and perspective. Structure is concerned with organizing the different parts of the image into a harmonious whole within the borders of the film frame. Dynamic lines are those within the subject image that are juxtaposed at exaggerated angles for visual impact, and are created by visualizing the image in a purely linear, two-dimensional abstract way. Finally, perspective produces the illusion of depth on a two-dimensional surface, enabling the viewer to differentiate size and distance in the image. The visual effects of lenses with different focal lengths vary enormously from the dramatic perspective enhancement of a wide-angle lens, to the compressed image produced by a long-focus lens.