Iris diaphragm

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The Iris diaphragm is a mechanical component of the lens which allows for changing aperture. It is made up of a set of blades which move outwards as the aperture is increased, either through the aperture ring, or by an electronic or mechanical instruction from the camera. When the lens is stopped down the blades are fully deployed, creating the smallest possible hole for the light to travel through.

The number and design of the blades is crucial to the boke of the lens.