On the following pages are examples of perspective from unusual viewpoints. When the observation point is too close, too high, or too low, the original assumption that all verticals are perpendicular to the ground is no longer valid. We know that when we stand at the base of a tall building and look up, the vertical lines seem to converge. The opposite effect is experienced when looking down from a window of an upper floor. The verticals are converging to a third vanishing point above or below the horizon line.
A graphic explanation of three-point perspective entails the use of a slanted picture plane. A professional photographer corrects converging vertical lines by using a swing front or PC lens (perspective correction). The lens tilts forward until parallel with the object, thereby correcting the image that falls on the negative. This is essentially what the graphics of perspective does to keep vertical lines perpendicular. It follows that if we want to find the convergence of a given situation, we tilt the picture frame forward to an angle approximately perpendicular to the center line of the cone of vision. If the situation is reversed, a high angle of vision, the picture plane is tilted back.
The slanted picture plane serves as an explanation of three-point perspective; however, the designer should feel free to vary the angle for design reasons. Many times a vertical, or third vanishing point, can be arbitrarily established in relation to right and left vanishing points to create an object or group of objects with a convergence that is right for the composition.