Diaphragm (optics)



In optics, a diaphragm is a thin opaque structure with an opening (aperture) at its centre. 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 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 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 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 human eye. The iris is the diaphragm, and the opening in the iris of the eye (the pupil) is the aperture. An analogous device in a photographic lens is called an iris diaphragm.

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. In modern cameras, an iris diaphragm is usually used; it has an adjustable opening, like the iris of the eye. Normally, the opening is shaped in a near-round fashion by a number of movable blades. Iris diaphragms usually have five to eight blades, depending on the intended uses, pricing and quality of the device in which it is used. Furthermore, each blade can be curved, resulting in an 'inflated' pentagon (or other polygon) shape, to improve the overall roundness of the iris opening.

Some modern automatic point-and-shoot cameras have a small set of selectable fixed diaphragms, rather than an iris diaphragm (for example, the Polaroid x530 has only two apertures).

The number of blades in an iris diaphragm has a direct relation with the appearance of the blurred out-of-focus areas in an image, also called Bokeh. The more blades a diaphragm has, the rounder and less polygon-shaped the opening will be. This results in softer and more gradually blurred out-of-focus areas.

In a photograph, the number of blades that the iris diaphragm has can be guessed by counting the number of 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.

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" (when the blades are able to be recessed into the sides of the lens, allowing the interior edge of the lens barrel to effectively become the iris).

History


In 1762, 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."

In 1867, Dr. Désiré van Monckhoven, in one 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 Rudolph Kingslake, the inventor of the iris diaphragm is unknown. Others credit Joseph Nicéphore Niépce for this device, around 1820. Mr. 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.