
148 Chapter 7
tip clipped off. Everything between the lens and
the furthest plane in the eld of view is the view-
ing area. The exact shape of the frustrum varies
depending on what focal length of lens (wide-
angle, normal or telephoto) is being used, and on
the aspect ratio of the sensor.
Lens Types
Prime lenses are lenses of a xed focal length. Zoom
lenses cover a range of focal lengths using movable
elements within the barrel of the lens assembly. In
cinema usage, a zoom lens must be parfocal, that is,
it must stay in focus as the focal length is changed. A
normal lens is a lens with a focal length close ...