The focal depth thing is related to a human vision system bug/feature - Your eyes want to change their 'vergence' at the same time they change focal distance. T

The focal depth thing is related to a human vision system bug/feature - Your eyes want to change their 'vergence' at the same time they change focal distance. This is sort of a hard-coded geometry solution. When you want something close your brain crosses your eyes a bit, adjusts the image to your brain, and changes focus. When you look far away your eyes uncross a bit, apply another transform to the result, and change focus again. Getting the eye-crossing and simultaneously trying not to change focus is one of the things that gives people eye strain and headaches when using VR.