Click 
to display the scene in the orthographic view.
Click 
to display the scene in the perspective view.
From the Perspective/Orthographic Settings 
, you can adjust the following:
Angle of view: Angle through which the viewer can see the visible world. As the angle of view gets narrower, objects appear closer to the viewer, thus larger in the viewport.
Focal length: Distance between the lens center and the image sensor, stated in millimeters. Changing the focal length changes the angle of view. The shorter the focal length (e.g., 18 mm), the wider the angle of view
Near and far planes are located at two particular distances from the camera position along the camera's view direction. Only the part of the scene between these two planes will appear in the viewport. Objects closer to the camera than the near plane or farther from the camera than the far plane will be clipped and not rendered.