Diaphragm (optics)
In
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 aperture and f-number 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
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.[2]
In the early years of photography, a lens could be fitted with one of a set of interchangeable diaphragms,[3] 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 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 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
Some modern automatic
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.[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:
- "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
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-Bergedorf location had a 60 cm (~23.6 inch) aperture Great Refractor by Reposold and Steinheil (Lenses).[10] One unique feature of Hamburg Great Refractor is Iris diaphragm that allows the aperture to be adjusted from 5 to 60 cm.[11] This telescope was activated in the early 1910s.[11]
See also
- Aperture
- f-number
- Shutter (photography)
- Leaf shutter
- Diffraction spike
References
- ^ Louis Derr, Photography for students of physics and chemistry London: The Macmillan Co., 1906
- ^ "Iris Diaphragms - Iris Diaphragm | Edmund Optics". www.edmundoptics.com. Retrieved 2019-11-04.
- ^ Derr, Louis (1906). "Photography for students of physics and chemistry".
- ISBN 978-0-452-28678-8.
- ISBN 9780871692597.
Cardano De subtilitate libri camera obscura.
- ^ Leonhard Euler, "Precautions to be used in the Construction of Telescopes. Necessitiy of blackening the Inside of Tubes. Diaphragms." 1762, in Letters of Euler on different subjects in physics and philosophy. Addressed to a German princess, Vol. II, Henry Hunter, D.D. (ed.), London, 1802,
- ^ Désiré van Monckhoven, Photographic Optics: Including the Description of Lenses and Enlarging Apparatus, English translation, London: Robert Hardwicke, 1867
- ^ Rudolf Kingslake, A History of the Photographic Lens, London: Academic Press, 1989
- ^ J. Henle, W, Keferstein, and G. Meissner, Bericht über die Fortschritte der Anatomie und Physiologie im Jahre 1867, Liepzip: C. F. Winter'sche Verlagshandlung, 1868.
- ^ "A SHORT HISTORY OF HAMBURG OBSERVATORY". www.hs.uni-hamburg.de. Retrieved 2019-11-04.
- ^ a b "The Hamburg Observatory" (PDF).