Some might remark that realism isn't as important here and being able to use more screen real estate on wide aspect ratio displays; in that case a clipped circle would offer more viewable area, still portray a more aesthetic realism, and portray a more consistent image/view across different displays -- two illustrations of that can be executed be seen below:
http://media.defenceindustrydaily.com/images/ELEC_NVG_SoF_Iraq_lg.jpg
http://www.chinook-helicopter.com/operations/hover/NVG_Master.jpg