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Comparing the size of the human pupil in bright and dark conditions. In these images from a medical scanner the pupil is outlined in red. In daylight the pupil is typically around 2 mm in diameter. In dark conditions the pupil expands to a diameter of approximately 5 mm in older adults and up to 7 mm in younger people, increasing the amount of light entering the eye by a factor of approximately ten times.

 1.1 Understanding dark adaptation