En Español: Relación de aspecto y área segura
What is aspect ratio?
In video processing, the aspect ratio is a pair of numbers that expresses the "wideness" of an image or video. The two numbers are a width-to-height ratio. For example, an aspect ratio of 4:3 means the picture is 4 units wide for every 3 units in height.
Typical 1980s-1990s TV sets have this aspect ratio, as do most computer monitors. In the early 2000s, High-definition TV (HDTV) screens have become more popular and affordable; Almost every HDTVs set have a 16:9 aspect ratio.
To find the aspect ratio of a video or image, just measure its width and height. The aspect ratio can then be expressed as width:height. If the picture is 24cm wide and 18cm tall, the aspect ratio can be expressed 24:18.
The aspect ratio is expressed in terms of the intended display dimensions. The resolution of an image in pixels is not always a good indicator of aspect ratio. For more on why this is so, see Aspect Ratios and Frame Sizes. How does 24:18 compare to other aspect ratios? Is wider or narrower than other aspect ratios like 16:9 or 4:3? It's hard to tell at a glance, because the denominators are different. The easiest way to compare aspect ratios is to reduce them to lowest terms by doing a simple division: 4/3 = 1.33, and 24/18 = 1.33, so 4:3 is the same as 24:18. Widescreen is 16/9 = 1.78. Reducing aspect ratios to their lowest terms makes comparison easier.
Two other aspect ratios are commonly used in filming movies: 1.85:1 (Academy Standard, or "flat"), and 2.35:1 (or "Scope", also called Panavision or CinemaScope). Both are even wider than 16:9, with Scope being almost twice as wide as 4:3. The term "widescreen" typically refers to any aspect ratio greater than about 1.33:1.
Converting between aspect ratios
Since the 1950s, theatrical releases have typically used a widescreen aspect ratio. Many Older TV sets (pre 2008) use a 4:3 aspect ratio, so widescreen movies must be converted for viewing on 4:3 TVs. Two popular approaches to this conversion are described here. They are pan and scan and letterbox.
With pan and scan, a 4:3 region is shown, cutting off the left and/or right sides of the original picture. A human operator (sometimes the director) selects areas that are important to the scene, and tries to fit them in the 4:3 frame. When the action moves to a different part of the screen, the visible region is panned over to it.
Throughout the 1980s, pan and scan was by far the most popular home video format. Pan and scan transfers make full use of the 4:3 frame, so the viewer's TV screen is filled at all times. In the 1980s, VHS still ruled the home market; pan and scan makes full use of VHS's low (by today's standards) resolution. By "zooming in" on the picture, detail and picture quality are better preserved.
But this gain comes at a significant loss; up to half of the original movie picture may be cut off. Camera shots framed for widescreen may look amateurish and asymmetrical when converted to pan and scan; sometimes, important characters, objects, or scenery may be lost. Some directors forbid pan-and-scan transfers of their films.
Note: Laserdisc was a popular niche market for movie buffs throughout VHS's reign; the picture quality of Laserdisc transfers is superior to that of VHS, but inferior to that of our beloved DVD.
See Widescreen vs. Pan & Scan Examples for some comparisons of widescreen and pan and scan on actual movies.
The letterbox format, in contrast, preserves the entire width of the original picture. Since the original film is not as tall (relatively speaking) as a 4:3 TV set, the areas above and below the picture are filled with black.
The advantage of letterbox format is that all of the original picture is visible. The disadvantage is that it's shrunk down to fit a 4:3 TV screen; a significant portion of the screen is "wasted" on the black bars. With VHS, a letterbox film would lose much of its original detail.
Since the late 1990s, pan and scan has become less popular. Its primary advantage to the consumer was optimal use of TV screen area, but the growing popularity of DVD, combined with the decreasing cost of HDTV, has made it easier than ever to enjoy high-quality widescreen at home. By the early 2000s, sales of widescreen (letterbox) and full-screen (pan and scan) DVDs were almost equal; some TV series have even begun using 16:9.
There are several other approaches to creating 4:3 versions of films, but those typically require special filming techniques, or access to final proofs of the original film. For more information on video-transfer methods, see How Film is Transferred to Video.
Pixels and aspect ratio
Images and videos stored on a computer are saved as a big rectangle full of tiny square pixels. Each pixel stores a single color; you can think of a digital image as a giant paint-by-number grid, with millions of possible colors. The more pixels in a digital image, the more detail can be captured. In the digital camera market, this resolution is simply expressed in megapixels, or millions of pixels. A 4.0 megapixel (4 million pixel) camera can capture an image roughly 2,000 pixels wide and 2,000 pixels high. It's a good thing computers are fast, because it would take a human a very long time to do a paint-by-number this big.
Digital video stored on a computer has no particular limitation on resolution, but digital video disc formats such as VCD, SVCD, and DVD only allow certain resolutions, in terms of pixels. The most common resolution for commercial DVDs is Full-D1, which is 720×480 pixels in size.
You may notice, if you try to calculate what aspect ratio this is, that 720/480 = 1.5, which is not a standard aspect ratio for either standard or widescreen TV sets. Obviously, the resolution of a video in pixels has little bearing on the aspect ratio of the video; 1.5:1 is the ratio of width to height in pixels, but nobody watches DVDs on a 1.5:1 TV set.
So why the weird ratio? There's a good reason for it. DVD was designed to perform well with both 1.33:1 (4:3 television) and 1.78:1 (16:9) widescreen). But instead of having two different pixel resolutions, DVD uses only 720×480. Since 1.5:1 is about halfway between 1.33:1 and 1.78:1, Full-D1 DVD is well-suited to encoding both of these common aspect ratios.
To achieve this, videos are first squished or stretched to fit into 720×480 pixels. Each video is marked with a bit of information telling the DVD player whether 4:3 or 16:9 should be used for playback. The DVD player then automatically does the appropriate squishing or stretching to restore the proportions to normal.
A 4:3 picture fills the viewing area of a 4:3 screen, but may be stretched or letterboxed when played back on a widescreen TV. DVDs using this format are typically labelled "full screen."
A 16:9 picture is letterboxed when played on a 4:3 screen, but fills the viewing area of a widescreen TV. DVDs using this format are typically labelled "widescreen."
In these two formats, all letterboxing is done by the DVD player (or, in some cases, by the TV). All 720×480 pixels are used optimally, with none wasted on black bars. With DVD, consumers can choose the aspect ratio that best suits their viewing needs.
Television sets are not perfect. Most TVs cut off a small amount of the picture around the edges, due to overscan. If there is important imagery or text present around the edge, it may not be visible on the average TV set. Directors and video producers keep important visual elements away from the edges by staying within the safe area.
In doing video production, a good rule of thumb is to keep important text inside a 90% "title safe" area, and less important elements within a 95% "action safe" area. Anything outside 95% is likely to be clipped off by normal TV sets.
If you want to find out your TV's safe area, download and burn a safe area test video. This video shows a growing green box and a percentage; when the green box goes off-screen, make note of the percentage (go frame-by-frame if necessary). This percentage is your TV's approximate safe area.
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