system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: My binoculars say "perma focus" on them and are in focus whether I'm looking at something 10m or 100km away. They don't have a centre focus wheel or individual eyepeice adjustments. How is this possible and why don't all binoculars do this?
gpt
This is possible if you block out all the light rays except those going through a small area in the centre of the lens. The centre of the lens is the best part of the lens and can focus light rays from all distances to the same point. 

This is why squinting helps you see in focus if you have bad eyesight. 

In an optical device, the downside is that you are blocking out and therefore losing most of the light and wasting your expensive lens. In daylight, this doesn't really matter because there is so much light. It only becomes an issue at night, twilight or if it's a very overcast day. 

You do also lose the in-focus / out-of-focus clues that aid you in judging distance.