Understanding Pixel Aspect Ratio

Posted on by Larry

[ This article was first published in the November, 2007, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe. ]

Of all the issues I get questions on, none are more frustrating than dealing with still images. So, returning us again to the issue that refuses to die, Chris Roberts writes:

I wonder if you can assist with a bit of a discussion I’ve been involved with on our user group emailing list. I remember when you gave your seminars here at the beginning of the year, there was some discussion about videos sizes and using anamorphic video in iDVD. At the time you recommended the best way to overcome this issue is to use QT Pro to resize the display size of the video, but changing the height not the width of the video, even though anamorphic images would normally be stretched horizontally. I hope I have this right.


Is it possible for you to just briefly describe why we should change the vertical size of the movies, rather than their width?

Larry replies: Chris, both NTSC and PAL use rectangular pixels to describe their images. The computer uses a square image. The shape of the pixel is it’s “aspect ratio,” that is the ratio of the length compared to height.

Still Images

Computers have an aspect ratio of 1 (1:1), the length and the height of the pixel are identical.

By default, we describe the height of a pixel as being equal to one. That means that the width of the pixel varies, depending upon format. For example, NTSC has an aspect ratio of 1:0.906. PAL has an aspect ratio of 1:1.06. Neither format is square, and both use differently-shaped rectangles; NTSC is tall and thin, while PAL is short and fat. The picture above illustrates this.

To get these images to look good on a computer monitor, say, when we export them as still frames for print or web use, we need to convert the aspect ratio from rectangular to square. This means adjusting the width of an image, because the height is always, by default, set to the correct length.

Just in case you were hoping that this issue goes away with HD, disappointment looms in your future, DVCPRO-HD, HDV, XDCAM-HD, and HDCAM all use rectangular pixels.


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