pixel aspect ratio

[iwantaehelp]

what is the pixel aspect ratio? and does it make a difference with your video quality?

[Mylenium]

Pixel aspect ratio is a concept from the olden days of video where hardware was simply not able to process large amounts of data/ signals and imagery had to be stretched and compressed to give the illusion. Similarly PAR is related to cinematic lenses often being anamorphic/ elliptical to squeeze e.g. very wide content onto the available film. I'm sure if you feel so inclined you could find lengthy articles on Wikipedia or other specialised web sites. Suffice it to say that PAR is a real thing and you have to at least understand some basics in case someone hands you over an old VHS tape or some anamorphic film imagery.

Mylenium 

[Rick Gerard · Accepted solution]

I'm going to add one thing to Mylenium's post. All of your compositions should be Square Pixels. The only exception to that rule is when you must render something that will be copied to an original Nonsqyare device like DV-NTSC or DVC-Pro. Every Nonsquare ratio aspect matches a Prspecific frame and frame rate. You cannot fiddle with the presets. There are few devices still left, and no new ones are produced that will record or playback a digital DV, DVCPRO, or any other old nonsquare format. The only modern use of a Nonsquare format is Anamorphic, and if you use that one, you have to match the rest of the footage and any other assets used in the production. 

There is no modern media player that will un-stretch a nonsquare render. If you send an NTSC DV-wide screen video to YouTube or social media, it will be 4:3 instead of 16:9, and everything will be fouled up. Circles will be ovals, and squares will be rectangles. 

If you drop Nonsquare footage in a square pixel comp, AE will automatically fix the problem, so there is nothing for you to do. 

[DayVess]

"There is no modern media player that will un-stretch a nonsquare render...Circles will be ovals, and squares will be rectangles."

Open up any video file in VLC Media Player, then start tapping the A key.

You'll see a very popular and up to date modern media player that can stretch and un-stretch a nonsquare render with ease.