How to keep HD Aspect Ratios when exporting to SD

Apple updated a little article today called "How to maintain aspect ratio when exporting high definition formats to standard definition".

It helps you figure out how to export an HD project to an SD file and still maintain it's proper aspect ratio. It basically says use a Reference Movie for non-IMX formats and create a new preset for IMX exports.

Calculating a 16:9 Ratio

Sometimes, clients want their 16 x 9 video at the correct aspect ratio but they want it smaller (or bigger...gah) and it's not always easy to know what an odd size for 16:9 is. Here's what I do.

16:9 is a ratio that's .5625

9 divided by 16 = .5625
9 divided by .5625 = 16
16 times .5625 = 9

You can think of it like this:

Small divided by Big = .5625
Small divided by .5625 = Big
Big times .5625 = Small

Or even:

Height divided by Width = .5625
Height times .5625 = Width
Width times .5625 = Height

So, knowing that, if they want a video that's say some weirdo size of 900 wide you can go 900 times .5625 and know that the height should be 506, so you set your encoder/transcoder to make it 900 x 506 (try and keep them multiples of two, btw. No one will notice if it's a pixel or two off.)

(In case you're curious 4:3 (ie..640 x 480) works the same way only the magic number is .75)