I hope you can help me out with this. I am exporting a come in AE, and even though my dimensions are set at 1920X1080, it keeps spitting out this 16:9 aspect ratio, stretching the whole video out. I've tried finding where to control this, but can't seem to export my video without stretching it.
Like Michael said it may be a PAR issue. Make sure that the comp you work in is 1920x1080 with a square pixel aspect ratio. Also, all your footage should be conformed to that. When you export you can choose a HD 1080 preset- just double check that it has a square pixel aspect ratio.