How do eye doctors know the correct prescription for corrective lenses for babies who need glasses?

The Dr. can shine a light in the eye, the instument used is called a retinoscope. They can tell what power the child needs by the way the light reflects off the retina and bounce back out with a specific light pattern. Then the Dr. can neutralize the power with the same instument by changing the power settings on the retinoscope. Usually they will dialate the child's pupils for the exam.

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from the lenses they have the children look into and the test they perform when the child has to put his chin on that lift and then they get a reading that only a doctor would know and can translate

well, its like this. As a parent raising an infant to a child you notice things and usually when it comes to the stage where they tend to watch things go around the room or try to grab at things. They can try and figure based on depth perception.

Of course its never exact, its a ~*guesstimate*~