Definition: Design of lens aperture control designed to cut out peripheral rays not required for image or application. * Consists of set of flat, opaque blades with curved cut-out positioned inside lens and pivoted so that they partially overlap to leave hole centred on lens axis: the area of the hole determines the lens aperture setting. * Setting the aperture is controlled by ring or other mechanism that changes the angle at which blades overlap.
Shaping the highs
The shape of the diaphragm hole is determined primarily by the number of blades, second by their shape: more blades, the nearer hole can be to circular. This may seem a dull and obvious fact but the shape of the hole has important consequences for the image. It determines the quality of the out-of-focus image especially highlights and it plays a part in controlling lens flare and internal reflections. Unfortunately, multiple blades are more difficult to manufacture and create more resistance against closing and opening. The current fashion for top-end lenses is to have circular apertures - hey, just like lenses from the 1930's.

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