There are two lens classes that can be used in film making – spherical and anamorphic. Spherical lenses are more common and have wider applications. A rectangular film gate or imaging sensor area acts as the field stop determining both the field of view (FOV) angular extents and the format aspect ratio. An increase in the horizontal FOV is achieved by using a shorter focal length lens also giving a corresponding increase in the vertical FOV.
Anamorphic lenses project a version of the image that is compressed along one dimension. These lenses distort the image, squeezing it horizontally while leaving the vertical aspect unaffected. They were originally designed so that wide format imagery would fully utilize the film area of standard 35 mm frames.
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The anamorphic lens consists of a regular spherical lens plus an anamorphic attachment or an integrated lens element. There are several methods to anamorph an image: crossed slits, a cylindrical system, or a prismatic system. The anamorphic element operates at infinite focal length, so that it has little or no effect on the primary lens focus it’s mounted on but still anamorphes (distorts) the optical field.