how well your eyes work together to focus on an object.
Cover test to check eye alignment.
During a cover test, your eye doctor will have you focus on a small object across the room and then he or she will cover each of your eyes alternately while you stare at the target.
While doing this, your eye doctor will assess whether the uncovered eye must move to pick up the fixation target, which could indicate strabismus or a more subtle binocular vision problem that could cause eye strain or amblyopia ("lazy eye"). The test is then repeated up close.

Πηγή: Optometrists Today
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