optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...

, a diaphragm is a thin opaque structure with an opening (

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...

) at its center. The role of the diaphragm is to ''stop'' the passage of light, except for the light passing through the ''aperture''. Thus it is also called a stop (an aperture stop, if it limits the brightness of light reaching the focal plane, or a field stop or flare stop for other uses of diaphragms in lenses). The diaphragm is placed in the light path of a

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

or objective, and the size of the aperture regulates the amount of light that passes through the lens. The centre of the diaphragm's aperture coincides with the

optical axis An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light pro ...

of the lens system. Most modern cameras use a type of adjustable diaphragm known as an iris diaphragm, and often referred to simply as an iris. See the articles on

and

f-number In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...

for the photographic effect and system of quantification of varying the opening in the diaphragm.

Iris diaphragms versus other types

A natural optical system that has a diaphragm and an aperture is the human eye. The iris is the diaphragm, the

pupil The pupil is a black hole located in the center of the Iris (anatomy), iris of the Human eye, eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing ...

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 price and quality of the device in which it is used. Straight blades result in

polygon In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed '' polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two ...

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 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

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 , commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...

Smooth Trans Focus or Fujifilm APD lenses. Some modern automatic

point-and-shoot camera A point-and-shoot camera, also known as a compact camera and sometimes abbreviated to P&S, is a still camera designed primarily for simple operation. Most use focus free lenses or autofocus for focusing, automatic systems for setting the exposu ...

s 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

depth of field The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera. Factors affecting depth of field For cameras that can only focus on one object dis ...

. 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).

History

In his 1567 work ''La Pratica della Perspettiva'' Venetian nobleman Daniele Barbaro (1514–1570) described using a camera obscura with a biconvex lens as a drawing aid and points out that the picture is more vivid if the lens is covered as much as to leave a circumference in the middle. In 1762,

Leonhard Euler Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries ...

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 Désiré Charles Emanuel van Monckhoven (1834–1882) was a Belgian chemist, physicist, and photographic researcher. He was also an inventor and author.Day, Lance ''et al.'' (1996) ''Biographical Dictionary of the History of Technology,'' p. 4 ...

, in one of the earliest books on photographic optics, draws a distinction betweens ''stops'' and ''diaphragms'' in photography, but not in optics, saying: :"Let us see what takes place when the stop is removed from the lens to a proper distance. In this case the stop becomes a diaphragm. : * In optics, ''stop'' and ''diaphragm'' are synonyms. But in photographic optics they are only so by an unfortunate confusion of language. The stop reduces the lens to its central aperture; the diaphragm, on the contrary, allows all the segments of the lens to act, but only on the different radiating points placed symmetrically and concentrically in relation to the axis of the lens, or of the system of lenses (of which the axis is, besides, in every case common)." 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. According to

Rudolf Kingslake Rudolf Kingslake (born Rudolf Klickmann; August 28, 1903 – February 28, 2003)http://www.osa-opn.org/Content/ViewFile.aspx?id=2425 was an English academic, lens designer, and engineer. Kingslake was born in London, England in 1903 as Rudolf ...

, the inventor of the iris diaphragm is unknown. Others credit Joseph Nicéphore Niépce for this device, around 1820. J. H. Brown, a member of the Royal Microscopical Society, appears to have invented a popular improved iris diaphragm by 1867. Kingslake has more definite histories for some other diaphragm types, such as M. Noton's adjustable ''cat eye'' diaphragm of two sliding squares in 1856, and the Waterhouse stops of John Waterhouse in 1858. The

Hamburg Observatory Hamburg Observatory (german: Hamburger Sternwarte) is an astronomical observatory located in the Bergedorf borough of the city of Hamburg in northern Germany. It is owned and operated by the University of Hamburg, Germany since 1968, although ...

-Bergedorf location had a 60 cm (~23.6 inch) aperture Great Refractor by Reposold and Steinheil (Lenses). One unique feature of Hamburg Great Refractor is Iris diaphragm that allows the aperture to be adjusted from 5 to 60 cm. This telescope was activated in the early 1910s.

References

{{DEFAULTSORT:Diaphragm (Optics) Photography equipment Optical components

Iris diaphragms versus other types

History

See also

References