Please note that much of this content is present in WikiPedia and we should try to consolidate the two.
The aspect ratio of a screen is its width-to-height ratio. Aspect ratio is independent of screen size. A television screen 14 inches wide and 10.5 inches high has the same aspect ratio of a television screen 25 inches wide and 18.75 inches high. This aspect ratio is 4:3, or 1.33:1 (1.33 times as wide as it is high).
When referring to the aspect ratio you generally refer either to the terms “4 by 3” or “16 by 9” for television; for film you refer to the aspect ratio as the numerical term, eliminating the decimal point and the “:1.” Therefore, you would refer to 1.85:1 as simply ”one-eight-five.”
A topic related to aspect ratio is that of Pixel aspect ratio
Cinema and television screens have used a number of different aspect ratios over the years. The following are the common ratios in use today:
- 1.33:1 aka 4:3. Standard for non-widescreen television.
- 1.37:1 (aka “Academy Standard Flat”) This is the frame size of standard 35mm sound film when using 4 perferations. While no films are distributed in this format many projects are shot in 1.37:1 and matted (or cropped) horizontally to fit other aspect ratios.
- 1.66:1 (aka "European widescreen") Matting the Academy Standard Flat on the top and bottom to achieve a wider aspect ratio derives this aspect ratio. Super 16mm is natively 1.66:1.
- 1.77:1 (aka 16:9) This is the standard ratio for widescreen television and all HDTV formats.
- 1.85:1 (aka "Academy Standard Flat Matted") This is one of two major projection formats in American cinema, derived from matting the Academy Standard Flat on the top and bottom to achieve a wider aspect ratio. This aspect ratio can be shot using a 3-perf 35mm camera without wasting any negative. 3-perf 1.85:1 is becoming very popular again in this age of the Digital intermediate.
- 2.35:1 This is usually achieved by matting the 4-perf academy standard 35mm frame. But this wastes a lot of the negative so many people prefer to shoot using matted 3-perf or 2-perf (2-perf was once known as Techniscope). 2-perf uses the least film but it's also the rarest so it's tough to find the cameras and the telecine facilities.
- 2.40:1 AKA Anamorphic. This is the widest format available for modern cinema. It uses standard 35mm sound film with the image squeezed horizontally using anamorphic lenses. When projected a special anamorphic lens unsqueezes the image. These anamorphic lenses have some disadvantages compared to normal lenses.