What are Aspect Ratios

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.

4:3 standard
The 4:3 ratio for standard television has been in use since television's origins and many computer monitors use the same aspect ratio. 4:3 is the aspect ratio defined by the Academy of Motion Picture Arts and Sciences as a standard after the advent of optical sound-on-film. By having TV match this aspect ratio, films previously photographed on film could be satisfactorily viewed on TV in the early days of the medium (i.e. the 1940s and the 1950s). When cinema attendance dropped, Hollywood created widescreen aspect ratios (such as the 1.85:1 ratio mentioned earlier) in order to differentiate their industry from the TV.

16:9 standard
16:9 (generally named as: "Sixteen-by-Nine" or "Sixteen-to-Nine") is the international standard format of HDTV as used in Australia, Canada, Japan, South Korea, and the United States, as well as in Europe on HDTV and non-HD widescreen television (EDTV) PALplus. Japan's Hi-Vision originally started with a 5:3 ratio but converted when the international standards group introduced a wider ratio of 5⅓ to 3 (=16:9), invented by Kerns H. Powers in 1984. The 1.78:1 aspect ratio was the compromise between the 35 mm US and UK widescreen standard (1.85:1) and the 35 mm European widescreen standard (1.66:1). Many digital video cameras have the capability to record in 16:9. Anamorphic DVD transfers store the information vertically stretched in a 4:3 aspect ratio; if the TV can handle an anamorphic image, it will horizontally decompress the signal to 16:9. If not, the DVD player can reduce scan lines and add letterboxing before sending the image to the TV. Wider ratios such as 1.85:1 and 2.40:1 are accommodated within the 16:9 DVD frame by additional black bars within the image itself. The European Union and Federation of Bosnia and Herzegovina have instituted the 16:9 Action Plan, just to accelerate the development of the advanced television services in 16:9 aspect ratio, both in PAL and also in HDTV. The Community fund for the 16:9 Action Plan amounted to €228 million.
In Europe 16:9 is used in the U.K. (Almost all Mainchannels are in 16:9), France (especially NRJ12and NRJ Hits), Germany (ARD and ZDF and partly some other private channeles as RTL Television or Pro7), Switzerland (TSI and SSR; now is available free-to-air anywhere in HDTV the Hi-Def channel HD Suisse), Italy (through mainly the Satellite broadcasting platform SKY Italia - in fact from march of 2008 SKY Italia will pass all of its channel of Cinema category in 16:9, changing the utility of relative channel SKY Cinema 16:9 - and sometimes Rai Sport Satellite), Greece (SKAI[1], not SKY), and in other countries.

21x9 (Cinemascope)
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 29x9 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.