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Bayonet adapter with a conical recess in which there is a perforated screen made of a metal foil into which a hole with a diameter of 0.09 millimeters has been etched (small black dot in the middle).
Optical enlargement of the photochemically etched metal foil aperture in a reflected light microscope ( diameter of the opening = 0.09 millimeters).

In optics and photography, a pinhole is a small, hole-shaped opening that can be used to create a faint image even without a lens. It is the basis of the pinhole camera or the camera obscura .

The finer the opening, the sharper the image generated, but also the fainter the light. As a pivoting photographic aperture , it used to be built into simple camera lenses in order to reduce the amount of light or increase the depth of field on sunny days ; today this is done with continuously adjustable lamellar panels .

Fine pinhole diaphragms are also used in confocal technology or in the Fourier lens when it is a question of the desired scattering or diffraction of tightly bundled light rays, or the generation of an ideal diffraction disk .

In X-ray and gamma-ray optics, pinhole diaphragms made of heavy metals such as lead or tungsten are the simplest technical solution for generating geometrically correct images with this dangerous radiation.

Pinhole diaphragms were also important in earlier time measurement. The oldest hourglasses from the 14th to 17th centuries consisted of two glass bulbs, between which the sand flowed through a perforated diaphragm made of metal or mica. With sundials , a small opening is often built into the shadow rod, the so-called nodus , which can be used to display the date and thus the current equation of time .

In the classic planetarium and in today's small room planetariums, the star field projectors consist of appropriately punched pinhole diaphragms - a principle that, conversely, is also used with the pinhole glasses .

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