iris diaphragm

Definition: Device located within a compound lens which controls amount of light let through the optics by adjusting its effective diameter. * Consists of set of curved blades set on a ring or cam so that the angle that the blades make to the ring can be changed, so covering more or less of the lens aperture.
The more, the rounder
The same of the aperture left by the iris diaphragm is more important than might be expected: it is not merely a hole, it shapes the image, particularly the out-of-focus image and it shapes any internal reflections. Experience has shown that the more circular it is, the better and obvious that needs more blades. Unfortunately, using more blades slows down any stop-down operation -- important in these days of SLRs shooting at ten frames per second. So you'll find circular holes largely on large-format and range-finder lenses. However, the growing recognition of the effect of irish shape on bokeh has led manufacturers to use 8 curved bladed irises in top lenses to produce near-circular apertures.

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