Wired by Liz Stenson 8/11/2016
That's because the key to sharpshooting isn't really about focusing on the target itself; it’s about aligning your mark with your firearm's front and rear sights. “The ability to hit the target is your ability to correctly hold those two alignments points up against the target,†Pilkington says. This is harder than it sounds—but shooting glasses make it easier, thanks to some clever optical trickery.
These optical accessories look complex because they are. Shooting glasses are endlessly customizable; a pair from Swiss manufacturer Champion’s Choice is made from nearly 100 different pieces. That said, even the most complicated rigs typically boil down to three basic components: a lens, a mechanical iris, and a series of blinders that dangle from the frame like charms on a bracelet.
The lens can be ground to a sharpshooter's optical prescription, but that's not really what it's for. An eye at rest would rather focus on a distant object than one that is near at hand; focusing on something in the foreground requires effort, and can lead to fatigue. Adding just a touch of lens power (+0.50 diopter, for the opticians in the house) to a sharpshooter's prescription can help her sighting eye bring her gunsights into focus and keep them there, even as she concentrates on aligning them with the target in the distance.
But the lens presents a tradeoff: Bringing a gun's sights into sharper focus can make the target go fuzzy. That's where the blinders and mechanical iris come in.
More:
https://www.wired.com/2016/08/olympic-sharpshooters-look-like-cyborgs/
