Aspect ratio is the ratio between the height and width of an image. In the early years of cinema and television, the predominate aspect ratio was 1.37:1. This means that the width of the image is 1.37 times the size of its height. This is the aspect ratio for 35mm film, the medium in which most movies are shot in.
Widescreen is an aspect ratio that was first introduced in the 1920s but didn’t gain popularity in movies until the 1950s, particularly to combat dropping attendance due to the introduction of television. Widescreen refers to any image that is wider than the 1.37:1 format. The most common widescreen aspect ratio is 2.39:1.
Anamorphic is the process of shooting and projecting a widescreen image on full screen film. Since 35mm is square, there are only two ways to shoot a widescreen image on it. You can shoot so that the width fills the screen, but unused black areas with be on the top and bottom of the image. The other way is anamorphic, which fills the frame vertically and compresses the image horizontally to fit the film frame. This uses all of the film frame, giving the image the highest resolution possible.
Anamorphic widescreen is achieved through the use of special lenses. The most popular of these lenses are manufactured by Panavision. If you wait until the end of the credits of a movie, you will most likely see the Panavision logo denoting that the film was shot with Panavision anamorphic lenses. These lens system features a series of lenses and mirrors that compress the width of the image to fit onto a square 35mm frame.
Since the image stored on the 35mm is compressed, another series of anamorphic lenses must be used to project the image an reverse the process, giving you the high resolution widescreen image that the filmmakers intended to shoot. Another form of anamorphic technology is also used on DVD’s that allow viewers to artificially squeeze the image to fit a 4:3 TV ratio or display the widescreen image with a letterbox on the top and bottom.