The aspect ratio of an image is the ratio of its width to its height, and this is especially important for video because the aspect ratio of standard HD and full HD is 16:9, which is wider than the 4:3 or 3:2 aspect ratio of stills cameras.
These ratios are simpler than they look. A 16:9 image is 16 units wide and 9 units high. It doesn't matter what these units are because that depends on the screen size and the resolution – the point is that the ratio stays the same.
Full HD is 1920 pixels x 1080 pixels while standard HD is 1280 x 720 pixels, but the ratio of width to height is 16:9 for both. In fact you can work it out with a calculator – if you divide the width (1920) by 16 and then multiply by 9 you get the height (1080), and you can do the same with standard HD.
DSLRs and compact system cameras with APS-C or full-frame sensors have a native aspect ratio of 3:2, while compact cameras and Micro Four Thirds cameras have an aspect ratio of 4:3. Both can shoot in the 16:9 ratio used for HD video, but only by cropping off the top and bottom parts of the image.
The new 4K and UHD video formats are in the 16:9 ratio too.
Watch out for 'anamorphic 16:9', though it's less common these days. Here, a 16:9 image is squeezed horizontally to fit a 4:3 sensor and then 'de-squeezed' when it's played back. This saves on bitrate and file sizes, but gives lower horizontal resolution.
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