You could almost certainly still be able to make the shot even if the screen is out of focus as long as you can put the crosshair into the silhouette where you want it to hit, but focus is the units' biggest drawback.
It's more akin to a camera, than a scope in that respect.
Think of it like this, you have your camera focused on a particular thing, the objects in the background or the foreground, are not going to be focused as well as what you are "looking" at.
I don't know any other way to describe it.