I actually took the time to calculate this a while ago - basically, angular resolution drops off pretty much exactly as 1/(angular distance) from the center of

I actually took the time to calculate this a while ago - basically, angular resolution drops off pretty much exactly as 1/(angular distance) from the center of focus, and the total angular size of the human visual field is only so big (and is measured). The actual answer is closer to 1 megapixel at any given time.