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Different moving picture displays have different shapes. The screens in movie theaters, for example, look like long rectangles; most TV screens and computer monitors are almost square. The shape of a video display is called the aspect ratio. The following two sections look at how aspect ratios affect your video work.
Image aspect ratios
The aspect ratio of a typical television screen is 4:3 (four to three) — for any given size, the display is four units wide and three units high. To put this in real numbers, measure the width and height of a TV or computer monitor that you have nearby. If the display is 32 cm wide, for example, you should notice that it's also about 24 cm high. If a picture completely fills this display, the picture has a 4:3 aspect ratio.
 | Different numbers are sometimes used to describe the same aspect ratio. Basically, some people who make the packaging for movies and videos get carried away with their calculators, so rather than call an aspect ratio 4:3, they divide each number by three and call it 1.33:1 instead. Likewise, sometimes the aspect ratio 16:9 is divided by 9 to give the more cryptic-looking number 1.78:1. Mathematically, these are just different numbers that mean the same thing. |
A lot of movies are distributed on tape and DVD today in widescreen format. The aspect ratio of a widescreen picture is often (but not always) 16:9. If you watch a widescreen movie on a 4:3 TV screen, you see black bars (also called letterboxes) at the top and bottom of the screen. This format is popular because it more closely matches the aspect ratio of the movie-theater screens for which films are usually shot. Figure 1 illustrates the difference between the 4:3 and 16:9 aspect ratios.
Figure 1: The two most common image aspect ratios.
A common misconception is that 16:9 is the aspect ratio of all big-screen movies. In fact, various aspect ratios for film have been used over the years. Many movies have an aspect ratio of over 2:1, meaning that the image is more than twice as wide as it is high! But for many films, 16:9 is considered close enough. More to the point, it's just right for you — because if your camcorder has a widescreen mode, its aspect ratio is probably 16:9.
Pixel aspect ratios
You may already be familiar with image aspect ratios, but did you know that pixels can have various aspect ratios too? If you have ever worked with a drawing or graphics program on a computer, you're probably familiar with pixels. A pixel is the smallest piece of a digital image. Thousands — or even millions — of uniquely colored pixels combine in a grid to form an image on a television or computer screen. On computer displays, pixels are square. But in standard video, pixels are rectangular. In NTSC video, pixels are taller than they are wide, and in PAL or SECAM, pixels are wider than they are tall.
 | Pixel aspect ratios become an issue when you start using still images created as computer graphics — for example, a JPEG photo you took with a digital camera and imported into your computer — in projects that also contain standard video. If you don't prepare the still graphic carefully, it could appear distorted when viewed on a TV. |
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