top-quality roof prism binoculars have eyecups that neatly slide
in and out, or they have twist up eyecups, letting the user
dial exactly the needed amount of eyecup extension. Twist-ups
also maintain their position without collapsing at crucial moments.
Most binoculars have folding eyecups. If you wear glasses, you should fold the eyecups down, like this. If you don’t wear glasses, make sure the eyecups are extended, like that.