I have a very BASIC question regarding pixel aspect ratio. I want to animate a music clip using multiple layers of photographs, all of which I have in high res. I'd like the clip to be HD 1920x1080. However I notice many questions from people who have set up their project in this format and then run into problems later when wanting to burn it to DVD. Why is this? Also if at some point I thought the clip may be used later down the track for TV broadcast (I am in PAL land) would HDV 1080 25 be unsuitable?
After Effects is resolution independent so you can create your comp at any size (as long as it is at least the size of your planned output). Once it's created you can then nest it into another comp which is the correct size for output. The dimensions of your comp are known as the Frame Aspect ratio. The shape of the actual pixels is what's determined by the pixel aspect ratio.
So, for example, you can create your comp at a frame aspect ratio of 1920x1080 but before outputting for DVD you would have to scale it down to "Widescreen NTSC DVD" preset (assuming you're NTSC? If not PAL equivalent). This has a frame aspect aspect ratio which is 720x480 with a 1.4 approx Pixel Aspect Ratio and 23.976 FPS.
You can then nest your 1920x1080 movie into a new NTSC DV Widescreen comp and scale or crop the nested video to fit into this comp before rendering.
If you have no need for the higher reolution file then it would be easier just to start with a DV Widescreen comp of course.
Hope this helps. Here's a primer on Aspect Ratios too: