What is an Aspect Ratio?

A ratio is essentially, a comparison. Ratios are expressed as "(value) by (value)" or a "(value) (value) ratio." An "aspect" ratio is used to describe the resolution of a video or graphics display, usually as part of a technical description of a particular piece of hardware or a device like a high-definition television (HDTV) or computer display.

It has been said that all software problems are inherently hardware problems, and this is nowhere more apparent than in the technical considerations of the current migration from analog to digital video displays. The benefits of digital broadcasting, for example, are undeniable, but every signal, regardless of its type or format, requires some piece of hardware to display it, and that's where aspect ratios become important.

Simply put, an aspect ratio is simply a comparison of the horizontal resolution of a display and the vertical resolution of the same display. Many current home computers, for example, are configured to an 800x600 pixel desktop display. The aspect ratio of this display is 4:3, which is a widely used standard. It means there are four pixels of horizontal display resolution for every three pixels of vertical resolution.

As the digital broadcast and display formats become more common, the 16:9 aspect ratio is becoming more popular, mainly because it is compatible with theatrical screen displays. Many new high-definition or "home theater" displays are constructed specifically to support this aspect ratio, meaning the physical dimensions of the device have the same ratio as the resolution, maximizing the efficiency of the display and avoiding letterboxing or pillarboxing, which leave blank areas for incompatible aspects.

Letterboxing is a way to "fit" a 16:9 aspect ratio signal inside the limited dimensions of a 4:3 device, such as a standard television. If a landscape photograph of a beachfront, for example, is being displayed at 1600x900 resolution, or a 16:9 aspect ratio, there is no way for it to fit inside a standard television unmodified. Because a standard television's physical dimensions are designed around a 4:3 display, the only way to make it fit is either to compress (and distort) the horizontal dimensions of the photograph, or leave the upper and lower thirds, roughly, of the display blank, because at full width, there isn't enough horizontal data in the photograph to fill the rest of the display horizontally.

Pillarboxing is the opposite. Displaying a 4:3 ratio signal on a 16:9 device means there will be insufficient horizontal information, so the left and right thirds of the display will be blank.

Aspect ratios of 16:9 are often called "widescreen" while a 4:3 aspect ratio is known as a "fullscreen" format. DVD movies often use these descriptions.