[Silverthorn]

I am currently following the "Generation Kill" serie on Historia and realized that almost everyone has some kind of rectangle "slit" made with tape in front of their scopes and binoculars (it turn the scope from a round view to a rectangle view, also dramatically reducing the field of view).

Why is that done? Is it to cut the glare of the sun? Is it to reduce the reflection that the enemy might see?

Thanks!