Your eye doctor also may use an autorefractor or aberrometer to automatically determine your prescription.
With both devices, a chin rest stabilizes your head while you typically look at a pinpoint of light or other image.
An autorefractor, like a manual refraction, determines the lens power required to accurately focus light on your retina. Autorefractors are especially useful in certain cases such as evaluating young children who may not sit still, pay attention or interact with the eye doctor adequately for an accurate manual refraction.
Studies have shown that modern autorefractors are very accurate. They also save time. The autorefraction takes only a few seconds, and the results obtained from the automated test greatly reduce the time required for your eye doctor to perform a manual refraction and determine your eyeglass prescription.
An aberrometer uses advanced wavefront technology to detect even obscure vision errors based on the way light travels through your eye. Aberrometers primarily are used for custom or wavefront LASIK vision correction procedures, but many eye doctors are now incorporating this advanced technology into their routine eye exams as well.
The eye doctor will give you a complete eye exam including the machine that he will use to automatically measure your prescription and the eye chart which he will combined with the machine to determine your accurate prescription.