Diaphragm
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| an 8-bladed iris diaphragm image by Dusty Medusa (Image rights) |
A diaphragm (or iris or iris diaphragm) is a mechanism in a camera that makes a variable aperture to control the intensity of light that passes through the lens. Along with shutter speed, this is what controls the exposure received by the film or image sensor.
A diaphragm may take many forms, from very simple devices for "point-and-shoot" film cameras consisting of just two notched pieces of metal, to more complex ones used in higher-quality cameras which have many blades arranged in a circle. This arrangement, also called an "iris" after the corresponding structure in the eye, creates a nearly-circular aperture whose size can be varied as needed. There may be as few as 5 blades to as many as 19. In cameras with a small number of diaphragm blades, the shape of the aperture itself (e.g. a hexagon) can often be seen in defocused points of light.
To transmit a particular light intensity, the physical diameter of the aperture must scale up with increasing lens focal length. Thus, apertures are expressed by the f-stop scale or "F-ratio," which is the focal length divided by the entrance pupil diameter provided by the diaphragm opening.
