612 x 459 is 4:3 which is the *exact* aspect of the iPad screen. The original iPad 1 & 2 and iPad mini are 1024×768 px (which is 4:3). The new Retina iPad’s are 2048×1536 px (still 4:3) so the customer is giving you the correct aspect. I assume the simulated iPad frame on their web site has a 612 x 459 area for the video to show through the middle. This makes sense to me.
The question is do they expect the video to fill the 4:3 screen or be 16:9 letterboxed? 16:9 video does display letterboxed on an iPad but the customer might want it to fill the screen in which case I would crop to 4:3. You can do this right in Compressor as long as it doesn’t crop important information on the sides.
~jr
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