Techniques using aperture 3.1. Changing blur with a handmade aperture 1. In the human eye, the aperture is referred to as the iris. In the center of the iris is a circular opening called the pupil. The diameter of the pupil contracts in bright environments and expands in dark ones, adjusting the amount of light that reaches the retina.
Eye color depends on the color of the iris.
If the surface of the iris has a low pigment density, eyes are blue. If pigment density is high, eyes will be brown or black. The diameter of the human iris is a concentric circle, so the pupil is always a circle. A cat's iris is a vertical slit and a horse's is a horizontal oval. The size of an SLR camera's aperture is controlled by the movement of several aperture blades that behave much like the human eye, changing the size of the circumference.