If you happened across the Olympic Air Pistol event for the first time, you might've been puzzled by the competitors' eyewear:

So those are mechanical irises. Shooting competitors fine-tune them to bring their gunsights into sharp focus. Here's an example of one by manufacturer Gehmann:

And the competition frames they sell, for those who don't wear eyeglasses:

If you're wondering why competitors don't just rely on prescription glasses, it's because your eyesight can actually change over the course of the day. You're dealing with both fatigue and changing light conditions. The adjustability of the mechanical iris allows you to compensate for that.

Lastly, it's worth noting that Oh Ye-jin, the South Korean competitor who took Gold in the 10 Meter Air Pistol event this year, eschews the irises.