Dilated pupil. Close-up of a human eye with a dilated pupil. See P420/326 for the same eye without dilation. Dilation of the pupil (mydriasis) is a normal response to dim lighting. The enlarged aperture allows more light into the eye, improving sensitivity but reducing depth of focus. It also occurs in response to release of adrenalin in the body (the fight fright flight" response), sexual arousal or the use of stimulant drugs such as amphetamine. Abnormal dilation of one or both pupils sometimes results from injury to the eye or damage to the optic nerve."