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The iris, as the aperture of the eye, controls the light incidence onto the retina. Its center remains open – this is the pupil.

Under bright conditions the iris reduces the diameter of the pupil and enlarges it when it is dark. Like this contrasts between bright and dark objects can be very well perceived. The pupil also influences the depth of focus just like in a camera. The depth of focus is the area in which close and distant objects can be seen clearly at the same time.