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To understand custom laser eye surgery works, one must first understand what standard laser vision correction does to the cornea.
Below is a diagram of a model eye, showin on the left the cornea and how it`s curved shape ideally focuses peripheral light rays and brings images into focus.
In a myopic individual, there is a mismatch between the shape of the cornea and the length of the eye, resulting in the premature focus of the external light rays before they reach the retina (back of the eye).
Standard laser vision correction flattens approximately ½ the cornea (only the central part) and thereby adjusting the primary focus point back onto the retina.
 Unfortunately, when you've only flattened the central part of the cornea, this can cause 'night halos' with vision at night. This happens because the pupil enlargens with dark environment, and like a camera aperture, more light from the peripheral cornea enters the eye. This part of the cornea (see diagram B) has not been lasered with standard treatment and creates night halos. Custom treatment treats the peripheral cornea (diagram C) and minimizes this side effect.
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