Ideally your eyeware should not influence your head position, nor obscure any part of the target - that's down to a consistent anchor point.
A few years ago I tried varifocals, and realised that a perfectly focussed centre could influence my head position, and become a sighting aid. I then considered that a horizon and vertical on my dominant eye lens would help with head position and alignment.
However you then need the frames to be in an absolutely consistent position on your head.
I was also told that any such innovations (and I've got a few more) would be banned.
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