There are two classes of lenses that can be used in the film making - spherical and anamorphic. Spherical lenses are more common and have wider application. In case of spherical lens rectangular film gate (or imaging sensor area) acts as the field stop which determines both the angular extents of the field of view (FOV) and aspect ratio of the format. An increase in the horizontal FOV is given by using a lens of a shorter focal length, which also gives a corresponding increase in the vertical FOV.
Anamorphic lenses project a version of the image that is compressed along one dimension, in the other words these lenses distort the image, squeezing it horizontally while leaving the vertical aspect unaffected. Anamorphic lenses were originally designed so that wide format imagery would fully utilize the film area of standard 35 mm frames.
Pic.1 Spherical and anamorphic lens. Image forming.
The anamorphic lens consists of a regular spherical lens plus an anamorphic attachment (or an integrated lens element) that does the anamorphosing. There are several methods to create anamorphosing of image - using of crossed slits, cylindrical system or prismatic system. The anamorphic element operates at infinite focal length, so that it has little or no effect on the focus of the primary lens it's mounted on but still anamorphoses (distorts) the optical field.