Diaphragm (optics)

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In optics, a diaphragm is a thin opaque structure with an opening (aperture) at its center. Diaphragm (optics)_sentence_0

The role of the diaphragm is to stop the passage of light, except for the light passing through the aperture. Diaphragm (optics)_sentence_1

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). Diaphragm (optics)_sentence_2

The diaphragm is placed in the light path of a lens or objective, and the size of the aperture regulates the amount of light that passes through the lens. Diaphragm (optics)_sentence_3

The centre of the diaphragm's aperture coincides with the optical axis of the lens system. Diaphragm (optics)_sentence_4

Most modern cameras use a type of adjustable diaphragm known as an iris diaphragm, and often referred to simply as an iris. Diaphragm (optics)_sentence_5

See the articles on aperture and f-number for the photographic effect and system of quantification of varying the opening in the diaphragm. Diaphragm (optics)_sentence_6

Iris diaphragms versus other types Diaphragm (optics)_section_0

A natural optical system that has a diaphragm and an aperture is the human eye. Diaphragm (optics)_sentence_7

The iris is the diaphragm, the pupil is the aperture. Diaphragm (optics)_sentence_8

In the human eye, the iris can both constrict and dilate, which varies the size of the pupil. Diaphragm (optics)_sentence_9

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. Diaphragm (optics)_sentence_10

An iris diaphragm can reduce the amount light that hits a detector by decreasing the aperture, usually with "leaves" or "blades" that form a circle. Diaphragm (optics)_sentence_11

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. Diaphragm (optics)_sentence_12

The iris diaphragm in most modern still and video cameras is adjusted by movable blades, simulating the iris of the eye. Diaphragm (optics)_sentence_13

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. Diaphragm (optics)_sentence_14

Straight blades result in polygon shape of the diaphragm opening, while curved blades improve the roundness of the iris opening. Diaphragm (optics)_sentence_15

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. Diaphragm (optics)_sentence_16

For an odd number of blades, there are twice as many spikes as there are blades. Diaphragm (optics)_sentence_17

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. Diaphragm (optics)_sentence_18

This is most apparent in pictures taken in the dark with small bright spots, for example night cityscapes. Diaphragm (optics)_sentence_19

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. Diaphragm (optics)_sentence_20

Similarly, out-of-focus points of light (circles of confusion) appear as polygons with the same number of sides as the aperture has blades. Diaphragm (optics)_sentence_21

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). Diaphragm (optics)_sentence_22

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. Diaphragm (optics)_sentence_23

A rounder opening produces softer and more natural out-of-focus areas. Diaphragm (optics)_sentence_24

Some lenses utilize specially shaped diaphragms in order to create certain effects. Diaphragm (optics)_sentence_25

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. Diaphragm (optics)_sentence_26

Some modern automatic point-and-shoot cameras do not have a diaphragm at all, and simulate aperture changes by using an automatic ND filter. Diaphragm (optics)_sentence_27

Unlike a real diaphragm, this has no effect on depth of field. Diaphragm (optics)_sentence_28

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). Diaphragm (optics)_sentence_29

History Diaphragm (optics)_section_1

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. Diaphragm (optics)_sentence_30

In 1762, Leonhard Euler 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." Diaphragm (optics)_sentence_31

In 1867, Dr. Désiré van Monckhoven, in one of the earliest books on photographic optics, draws a distinction betweens stops and diaphragms in photography, but not in optics, saying: Diaphragm (optics)_sentence_32

Diaphragm (optics)_description_list_0

  • "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.Diaphragm (optics)_item_0_0
  • * 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)."Diaphragm (optics)_item_0_1

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. Diaphragm (optics)_sentence_33

According to Rudolf Kingslake, the inventor of the iris diaphragm is unknown. Diaphragm (optics)_sentence_34

Others credit Joseph Nicéphore Niépce for this device, around 1820. Diaphragm (optics)_sentence_35

Mr. J. H. Brown, a member of the Royal Microscopical Society, appears to have invented a popular improved iris diaphragm by 1867. Diaphragm (optics)_sentence_36

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. Diaphragm (optics)_sentence_37

The Hamburg Observatory-Bergedorf location had a 60 cm (~23.6 inch) aperture Great Refractor by Reposold and Steinheil (Lenses). Diaphragm (optics)_sentence_38

One unique feature of Hamburg Great Refractor is Iris diaphragm that allows the aperture to be adjusted from 5 to 60 cm. Diaphragm (optics)_sentence_39

This telescope was activated in the early 1910s. Diaphragm (optics)_sentence_40

See also Diaphragm (optics)_section_2

Diaphragm (optics)_unordered_list_1


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Diaphragm (optics).