Diaphragm (optics)

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.

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.

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.

[5] In 1762, Leonhard Euler[6] says with respect to telescopes that, "it is necessary likewise to furnish the inside of the tube with one or more diaphragms, perforated with a small circular aperture, the better to exclude all extraneous light."

In 1867, Désiré van Monckhoven, in one of the earliest books on photographic optics,[7] draws a distinction betweens stops and diaphragms in photography, but not in optics, saying: This distinction was maintained in Wall's 1889 Dictionary of Photography (see figure), but disappeared after Ernst Abbe's theory of stops unified these concepts.

[10] One unique feature of the Hamburg Great Refractor is an iris diaphragm that allows the aperture to be adjusted from 5 to 60 cm.