The term Aspect Ratio refers to the width of a picture (or screen) in relation to its height. Ratios are expressed in the form "width x height". For example, a 4x3 ratio means the picture is 4 units wide by 3 units high. Alternatively a colon may be used (e.g. 4:3 or 16:9) or a ratio to the number 1 (e.g. 1.33:1 or 1.78:1).
Note that the actual physical size of the picture is irrelevant — aspect ratio refers only to the relationship between width and height.
The three most common aspect ratios are shown below. There are many variations in addition to these but most video and film production uses one of these formats.
This is the standard television format used throughout the second half of the 20th Century. Sometimes referred to as 12x9.
This format has gained acceptance as the new standard for widescreen TV, DVD and high-definition video.
A very wide screen format used for theatrical release movies.
Converting Between Aspect Ratios
The problem of converting pictures between different formats has plagued film and television companies for years. Conversions almost always involve compromise and often annoy the end user, the film director, or both.
For example, if we take the 21x9 image above and convert it to a narrower format, we will have to lose quite a large part of the picture. The blue lines show where the picture will be cropped at 16x9, and the red lines show a 4x3 version.