Why does the optician cover one of my eyes?
This part of the test is called the divergence/convergence test.
The alignment of the eyes is observed, and the eyes are alternately covered and uncovered while the person looks at a target.
This tests the eyes' resting positions and their response to focusing and targeting on an object.
If your eyes are misaligned it might indicate a squint, an uncorrected optical error or even a neurological problem.