From The TV IV
Aspect ratio is the relationship between the width and height of a television image. All of the early TV systems shared the same aspect ratio of 4:3 which was chosen to match the format used in cinema films at the time.
In the 1950s, movie studios moved towards widescreen aspect ratios in an effort to distance their product from television. Although this was initially just a gimmick widescreen is still the format of choice today and square aspect ratio movies are rare. Some people argued that widescreen is actually a disadvantage when showing objects that are tall instead of panoramic, others would say that natural vision is more panoramic than tall, and therefore widescreen is easier on the eye.
Aspect ratios compared
Here is a comparison between the 4:3 standard fullscreen format and the 2.35:1 anamorphic widescreen video format taken from the movie "Star Trek: Insurrection". As you can clearly see, much of the image is lost for the fullscreen version, and there is nothing that can be done to prevent the loss of image on standard television, which is one of the reasons that you can find the "This film has been modified from its original version, it has been formatted to fit this screen." message at the beginning of films for TV.
|Fullscreen (1.33:1)||Anamorphic Widescreen (2.35:1)|
Pan and scan is a method of adjusting widescreen film images so that they can be shown within the proportions of an ordinary TV screen, by cropping off the sides of the original widescreen image. This is considered destructive to the directors original vision and intentions, because it can remove up to 45% (on 2.35:1 films) of the original image, and hinder the viewer's understanding of what's happening. This is the method for showing the fullscreen image on the left, and as you can see, about 45% of the picture is cropped out.
Here are examples for the two most commonly used aspect ratios. For fullscreen there is 4:3, that is three parts down by four parts across. But to say a picture is always four by three parts makes it hard to make exact measurements. Instead an alternative system is used; we measure the height and the width. For example, in the image below we have a ratio of 1.33:1 (or 1.33 to 1). The 1 can be anything. Imagine, for example, the blue rectangle below was 1 meter high. The width would then be 1.33 meters across, or, if you like: 1 meter and 1 meter 33 centimeters wide.
There are a myriad of possible ratios for widescreen. But probably the most used of them for DVD's will be 1:85:1, 1.66:1 (often used for Disney animation), 1.78:1, and especially CinemaScope at 2.35:1. CinemaScope is also sometimes called Panavision or just WideScreen.