How to use "focus" in a sentence

She hears the train in the dark tunnel and tells herself to wake up and to focus for another forty minutes.

The focus of all the seats was the immense stage, a marvel of the size required for a Las Vegas spectacular, more than half again as large as the largest stage on Broadway

She allowed her eyes to swim out of focus.

She put the pen at the edge of the map once more, and she let her eyes drift out of focus.