Nobody can tell you anything without proper technical details like exact comp and render settings.
First question, how did you render the file? Second question, what were the render settings? Third question, how do you know the picture aspect ratio is 21:9? My guess is that you have modified the output module, picked the wrong preset or format to render, or modified the output module or media encoder presets in some way. We can't know unless you explain your workflow and tell or show us your composition settings and render settings and show us the 411 on your rendered file. You can do that by importing the rendered file to AE, selecting the file in the Project Panel and reading the file specs there. The size and frame rate and PAR should match your composition. If they do not then you have fouled up the workflow.
Steps to follow for a successful render:
- Use the Presets in the New Composition settings to create your comp. Most folks should use HDTV 1080 29.97 or if you are in Europe HDTV 1080 25. If you must use 24 fps make sure you know how to handle the motion artifacts that horizontal and vertical motion may cause. Don't mess with the frame rates unless you absolutely know what you are doing. Do Not use non square pixels unless you absolutely know what you are doing because no modern delivery format is non square pixels
- Make sure that you do your final composition previews using the ACTIVE CAMERA view. That's the only view that will render.
- Render you DI (digital intermediate) or Production Format copy using the Render Cue. DO NOT edit the Render Settings from the factory defaults unless you know exactly what you are doing. Choose the default Lossless or Lossless with Alpha from the Output Module presets and enable audio if you need to in the output module. Do not edit the output module settings listed in the pulldown unless you know exactly what you are doing. To render your final compressed product for playback use the Adobe Media Encoder.
- When you use the Adobe Media Encoder the most common format is h.264. Use the presets available here that match your composition's frame rate and frame size. The YouTube or Vimeo HD presets are suitable for almost all delivery by the internet. Don't mess with them unless you know exactly what you are doing. The mp4 that will be produced by choosing h.264 and using a YouTube preset is the most universally acceptable format available today.
To learn more about rendering and delivery just type Rendering in the Search Help field at the top right corner of After Effects and study up. The combination of things you can foul up if you do not have a good understanding of video formats and standards is so huge that most people that modify settings by assuming they know what they want foul things up. Just check the forums. Just about every day at least one person asks why their movie is fouled up.
Check quicktime aspect ratio settings.Use other player to play and check.
Thanks for the help, but I couldn't wait, and figured it out myself.
It involved the whole render settings thing. I fixed it, but sadly, I don't remember what settings I used, lol. I probably should've wrote it down.
But I have it as an .mp4 file, and I forgot what I did, but it looks much better now!