Guides

The ratio of width to height of a displayed picture is called the aspect ratio or AR. AR can be listed as a ratio of integers like 4:3 (standard television) or 16:9 (widescreen television). More commonly they're listed as a ratio of something to 1 like 1.33:1 (4:3) or 1.78:1 (16:9). They can even be written with an implied 1 like 1.33 or 1.78. The last two notations are generally the most useful since they allow you to easily see the difference between multiple ratios.

In the early days of film a standard aspect ratio of 1.37 was established. This was the standard AR for all films until television started becoming popular. In order to combat the perceived threat of the new technology, with an AR of 1.33, movie studios figured out how to change the AR of a film frame without requiring new projectors to display it. This new technology, anamorphic lenses, only required that a special lense be used that squeezed a wide image into the space normally occupied by a standard 1.33 image. This looks wrong on the film itself, but when projected through an anamorphic lense that matches the one used for filming it returns to the wide AR of the original.

A standard television screen has an AR of 1.33, nearly the same as film's original 1.37 AR. These aspect ratios are considered fullscreen. Anything wider is considered widescreen. Other common aspect ratios include 1.66, 1.85, 2.21, and 2.35. Some movies, particularly epic movies released during the peak of widescreen experimentation in the 1950s, have aspect ratios as wide as 2.77.
Since different sources with different aspect ratios may need to be displayed on the same screen (which will obviously keep the same AR all the time), it's necessary to change the video's AR or add borders to two sides to avoid distortion. Some conversions can be done by simply stretching or squeezing it a small amount to match a particular display. This is how fullscreen film is converted to fullscreen television (1.37 to 1.33) and usually how 1.85 widescreen is converted for viewing on a 1.78 widescreen television. Video with a significantly wider aspect ratio than the display they're intended for is generally letterboxed. This adds borders to the top and bottom.

Anamorphic Encoding

Possibly the biggest misunderstanding in digital video is how anamorphic encoding works. Since video is often encoded for viewing on both fullscreen and widescreen displays it's necessary to try to optimize for widescreen while maintaining the highest quality possible for fullscreen playback. This can be done using anamorphic encoding. Like anamorphic lenses for film, anamorphic video is stored squeezed into a narrower aspect ratio than the original image. The image can then be stretched to the correct AR when it's displayed on a widescreen television. If, on the other hand, it's being viewed on a fullscreen display, it can be squeezed vertically instead. Since the human eye is more sensitive to vertical resolution than horizontal, anamorphic widescreen will retain more quality (visual resolution) than simple letterboxing.

Open Matte

Another technique that can help prepare a source for display in more than one aspect ratio is filming with an open matte. A matte is simply a border used to change the AR of film. It's an alternative to using an anamorphic lense. It requires a wider filmstock to capture the same area as an anamorphic lense. Using an open matte means that the entire film frame is exposed, with the matte applied when a print is made on a standard fillm frame. The benefit of an open matte is that prints with different aspect ratios can be made, and in many cases they even allow fullscreen transfers by applying no matte at all. This gives a different picture than the original film since it likely includes elements that the director may not have even been aware of (since they weren't intended for the film) but it's better than the alternative of Pan and Scan.

Pan and Scan

There are a few different names used to refer to pan and scan transfers. Most of them are less than flattering. The problem with pan and scan is that it only shows part of an original film frame in order to keep the AR correct to fill the entire (1.33) television screen. This causes major problems in frames where important action occurs at opposite ends of the screen. In many cases a pan and scan transfer also includes some tricks that remove or distort the middle of the frame in order to get the sides both into view at the same time. The general rule of thumb is that a letterbox or open matte transfer is better than pan and scan, and anamorphic is better than either.


Version History

v1.0 2007.08.26 Original version by Rich Fiscus
v1.0.1 2007.08.24 by Rich Fiscus
v1.0.2 2007.11.16 Added links to additional DVF guides by Rich FiscusTable of Contents

1. Introduction
2. Definitions
3. Resolution of Common Formats
4. Aspect Ratio