Refractive errors, like most of the things that optometrists measure, can be determined objectively and subjectively. Objective tests require little or no contribution from the patient. Subjective tests require a response from the patient, but invariably include an element of "double-checking." It should be stressed that children of any age (including infants) and of any capabilities (even with severe intellectual disabilities) can undergo an eye examination with an optometrist.
The principal objective technique for determining the refractive error uses a handheld instrument, the retinoscope, to shine light through the pupil, and to neutralize (with lenses) the rays that are reflected back from the retina. Eye care practitioners rely more on objective techniques when examining younger and less reliable children; patient cooperation is not essential for such an eye examination. Sometimes, eye drops are used to relax the accommodative mechanism and thus reveal the full refractive error. Modern drugs only sting a little, and the duration of blurred vision is just a few hours (the pupils remain dilated for longer so that sunglasses may still be needed on the day after the drops are inserted).
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