You can define perspective views of a model to create realistic effects. The difference between defining perspective views and defining parallel projections is that for perspective views, you must specify a distance between a theoretical camera and target point. Small distances produce severe perspective effects; large distances produce mild effects.
The following figure shows the same wireframe model in a parallel projection and in a perspective view. Both are based on the same viewing direction.
Many operations are not available in perspective view, including panning and zooming, and operations that require using object snap or input from the pointing device.
A perspective view remains in effect until the perspective effect is turned off or until a new view is defined in its place.