Left: an outside observer looking at the scene and the camera's view volume (drag to orbit, scroll to zoom). Right: what that camera sees. Companion to notes/06.
Shrink the field of view and watch the frustum narrow while the right image zooms in. Move the near plane until it cuts through a box. Switch to orthographic and notice that distant boxes stop shrinking. The red, green and blue arrows at the camera are its u, v and n axes: the rows of the view matrix shown in the panel.