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Understanding aspect ratio
In the video world, we often hear about aspect ratios. But it seems they are everywhere: there are ratios for lenses, frames and pixels. Are these all the same? The answer is no, but the different ratios you will encounter are highly correlated one between the other. In fact, they all apply to different stages in the process of acquiring digital video.
1. Shooting the video: camera lenses ratios
When you first acquire footage through your camera, you usually have a lens mounted on it…(!) Most people will have a regular 4:3 lens, which means than an image captured by it will have a length 1.33 times its height. Now as you probably know, it is also possible to shoot video in widescreen format, like what you see in theatres, which often implies the use of different lenses. To sort out all these possibilities, let's use the following example. You are on vacation in Copenhagen, and you have an NTSC DV camera that has a 16:9 ("widescreen") option. You are in front of the following scene and you would like to shoot it for a few seconds.
The red rectangle is the area the camera will take if you are using a regular lens, with or without the 16:9 option. The blue rectangle is the area you will get if you use a wide-angle lens or an anamorphic-squeeze lens. But what you will get on your TV is slightly different. Let's examine all four cases.
Regular lenses have an aspect ratio of 4:3. In that case, the image captured
by the camera is unaltered and is directly what you obtain on a TV:
The image on the TV has the same aspect ratio than the camera lens, 4:3. The true vertical resolution of the image is 480 lines since the entire frame is covered (the NTSC standard is used in this article; for PAL it would be 576 lines).
The aspect ratio of the lens does not change by activating the widescreen option of your camera. Instead, the image is trimmed to match a widescreen aspect ratio (16:9) and then "matted" with black bars, so that it looks like this on your TV:
You can observe that there is no increase in the viewing width, yet some
of the image at the top and bottom have been cropped. There, the frame
ratio is still 4:3, though the actual vertical resolution is 360 lines
(black bars do not count), so less than previously.
With these lenses, the wide-aspect footage is fit horizontally into a 4:3 frame and the leftover space above and below is filled black, looking like this on your TV:
As you can see, the viewing width is increased, now showing more of the boats and houses, while preserving the whole height of the picture. The vertical resolution, though, is still the same as before, 360 lines, less than the full 4:3 picture.
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