In a camera, a mechanism coupled to the shutter release that controls the lens diaphragm. The diaphragm closes to a preset value before the shutter opens, then returns to the open position once the shutter closes.
A natural optical system that has a diaphragm and an aperture is the human eye. The iris is the diaphragm, the pupil is the aperture. In the human eye, the iris can both constrict and dilate, which varies the size of the pupil. Unsurprisingly, a photographic lens with the ability to continuously vary the size of its aperture (the hole in the middle of the annular structure) is known as an iris diaphragm.
An iris diaphragm can reduce the amount of light that hits a detector by decreasing the aperture, usually with "leaves" or "blades" that form a circle.
In the early years of photography, a lens could be fitted with one of a set of interchangeable diaphragms, often as brass strips known as Waterhouse stops or Waterhouse diaphragms. The iris diaphragm in most modern still and video cameras is adjusted by movable blades, simulating the iris of the eye.
The diaphragm has two to twenty blades (with most lenses today featuring between five and ten blades), depending on the price and quality of the device in which it is used. Straight blades result in polygon shape of the diaphragm opening, while curved blades improve the roundness of the iris opening. In a photograph, the number of blades that the iris diaphragm has can be guessed by counting the number of diffraction spikes converging from a light source or bright reflection. For an odd number of blades, there are twice as many spikes as there are blades.
In the case of an even number of blades, the two spikes per blade will overlap each other, so the number of spikes visible will be the number of blades in the diaphragm used. This is most apparent in pictures taken in the dark with small bright spots, for example, night cityscapes. Some cameras, such as the Olympus XA or lenses such as the MC Zenitar-ME1, however, use a two-bladed diaphragm with right-angle blades creating a square aperture.
Similarly, out-of-focus points of light (circles of confusion) appear as polygons with the same number of sides as the aperture has blades. If the blurred light is circular, then it can be inferred that the aperture is either round or the image was shot "wide-open" (with the blades recessed into the sides of the lens, allowing the interior edge of the lens barrel to effectively become the iris).
The shape of the iris opening has a direct relation with the appearance of the blurred out-of-focus areas in an image called bokeh. A rounder opening produces softer and more natural out-of-focus areas.
Some lenses utilize specially shaped diaphragms in order to create certain effects. This includes the diffusion discs or sieve aperture of the Rodenstock Tiefenbildner-Imagon, Fuji, and Sima soft focus lenses, the sector aperture of Seibold's Dreamagon, or the circular apodization filter in the Minolta/Sony Smooth Trans Focus or Fujifilm APD lenses.
Some modern automatic point-and-shoot cameras do not have a diaphragm at all and simulate aperture changes by using an automatic ND filter. Unlike a real diaphragm, this has no effect on the depth of the field. A real diaphragm when more-closed will cause the depth of field to increase (i.e., cause the background and the subject to both appear more in-focus at the same time) and if the diaphragm is opened up again the depth of field will decrease (i.e., the background and foreground will share less and less of the same focal plane).