ANAMORPHIC WIDESCREEN: 35MM - 2.35:1 (21:9)
Anamorphic is a type of optical distortion used in image acquisi-tion (e.g. motion picture photography) designed to produce a widescreen (2.35:1) image, while maintaining optimal image quality from standard 35mm film stock. During recording, a special lens is used that squeezes the image horizontally. Then, during post-processing or presentation the image is stretched horizontally to restore the image back to normal.
The reason for this format's existence, lies in its history. While the widescreen format offered a new way to watch films, it had to be recorded on standard 35mm film in order to remain economically viable. This meant that the horizontal size of the image on the negative of the film was fixed and in order to produce the widescreen image, the vertical size had to be reduced. By contrast, in theaters it was the vertical size of the screen that had to be maintained in order to produce the desired "wrap-around" effect, thus an increased horizontal size was needed. These two needs, by the cinematographer and the projectionist were, as you can see, in direct conflict. For the cinematographer it was unreasonable to ask that the vertical image size be reduced because it meant that in the theater a larger blow-up would be needed to maintain the vertical screen size and this would reduce the over-all image resolution. And for the financiers it was unreasonable to increase the horizontal size and create a new film format that was wider than 35mm film because of the very large costs involved. Many methods attempted to strike a compromise between these two needs (VistaVision, Cinerama, etc).By optically squeezing the image during recording, you are capturing an image that is wider than it is tall (2.35:1 to be exact) while sacrificing none of the precious vertical size that is needed for optimum resolution. This image can then easily be stretched by the projectionist and the widescreen effect is preserved for a more aesthetically pleasing wrap-around effect. Nowadays, many films are simply cropped in vertical size to produce the widescreen effect; this is made possible because film stocks have improved to a degree where the argument for anamorphic images is made largely moot.
SOURCES:
http://en.wikipedia.org/wiki/Anamorphic