The aperture in the iris, the coloured area of the eye. The iris is a diaphragm which opens and closes to vary the amount of light admitted to the eye. Pupil size varies between 2.5 mm and nearly 9 mm, depending on the light conditions and the individual. The maximum, dark-adapted pupil size decreases with a person’s age from an average of 7 mm to about 5 mm. Observers should avoid using low-power eyepieces which give an exit pupil larger than their eye can accept, thus wasting light.