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A pinhole camera is a camera without a lens. The light producing the image passes through a small hole. In order to produce a reasonably clear image, the aperture has to be a small pinhole on the order of 0.02 inches (0.5 mm) or less. The shutter of a pinhole camera usually consists of a hand operated flap of some light-proof material to cover and uncover the pinhole. Pinhole cameras require much longer exposure times than conventional cameras because of the small aperture; typical exposure times can range from 5 seconds to more than an hour.

The image may be projected on a translucent screen for real-time viewing (popular for viewing solar eclipses; see also camera obscura), or can expose film or a charge coupled device (CCD). Pinhole cameras with CCDs are sometimes used for surveillance work because of their small size.

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Selection of pinhole size

Generally, a smaller pinhole will result in better image resolution (sharper picture) as the projected (circle of confusion) is smaller at the image plane. An extremely small hole, however, can produce significant (diffraction) effects which will result in a less clear image. Additionally, as the diameter of the hole approaches the thickness of the material in which it is punched, significant vignetting at the edges of the image will result, as less light will reach these areas. This is due to the shading effect of the sides of the hole for light coming in at other than a 90 degree angle.

The best pinhole is perfectly round (to minimise any higher-order diffraction effects of irregularites), and in an extremely thin piece of material. Industrially produced pinholes have access to laser etching, but a hobbyist can still produce pinholes of sufficiently high quality for photographic work.

One often quoted method is to start with a sheet of brass shim or metal reclaimed from an aluminium drinks can, use fine sand paper to reduce the thickness of the material in the center to the bare minimum, before carefully creating a pinhole with a suitably sized needle - sanding away the burrs on either side & rotating the pin as it glides in and out in order to produce a smooth circular hole.

A method of calculating the optimal pinhole diameter was first attempted by Jozef Petzval. The formula was improved upon by Lord Rayleigh into the form used today:

d=1.9√¯(f λ)

Where d is diameter, f is focus length (distance from hole to photographic film) and λ is the wavelength of light.

For standard black and white film, a wavelength of light corresponding to yellow-green (550 nm) should yield optimum results.

The depth of field is basically infinite, but this does not mean everything will definitely be in focus. Depending on the distance from the aperture to the film plane, the infinite depth of field means everything is either in or out of focus to the same degree.

The image is inverted, as shown in the diagram above.

Pinhole camera construction

Pinhole cameras are usually handmade by the photographer for a particular purpose. In its simplest form, the photographic pinhole camera consists of a light tight box with a pinhole in one end, and a piece of film or photographic paper wedged or taped into the other end. A flap of cardboard with a tape hinge can be used as a shutter. The pinhole is usually punched or drilled using a sewing needle or small diameter bit through a piece of tinfoil or thin aluminum or brass sheet. This piece is then taped to the inside of the light tight box behind a hole cut through the box. An oatmeal box can be made into an excellent pinhole camera.

Pinhole cameras are often constructed with a sliding film holder or back so that the distance between the film and the pinhole can be adjusted. This allows the angle of view of the camera to be changed and also the effective f-stop ratio of the camera. Moving the film closer to the pinhole will result in a wide angle field of view and a shorter exposure time. Moving the film farther away from the pinhole will result in a telephoto or narrow angle view and a longer exposure time.

Pinhole cameras can also be constructed by replacing the lens assembly in a conventional camera with a pinhole. In particular, compact 35mm cameras whose lens & focusing assembly has been damaged (smashed lens, dropped in sand etc.) can be reused as pinholes - maintaining the use of the shutter & film wind on mechanics. As a result of the enormous increase in f-stop while maintaining the same exposure time, one must use a fast film in direct sunshine.

Calculating the f-stop & required exposure

The f-stop of the camera may be calculated by dividing the diameter of the pinhole into the focal length of the camera. The diameter of the pinhole can be determined by knowing the diameter of the needle or drill used to make the hole. The focal length is the distance from the film to the pinhole.

For example, a camera with a 0.02 inch (0.5 mm) diameter pinhole, and a 2 inch (50 mm) focal length would have an f-stop of 2/0.02 (50/0.5), or 100.

Due to the large f-number of a pinhole camera, exposures will often encounter reciprocity failure. Once exposure time for film has exceeded 1 second, or that of paper has exceeded 30s - one must compensate for the breakdown in linear response of the film to intensity of illumination by using longer exposures.

Other special features can be built into pinhole cameras such as the ability to take double images, by using multiple pinholes, or the ability to take pictures in cylindrical or spherical perspective by curving the film plane.

These characteristics could be used for creative purposes. Once considered as an obsolete technique from the early days of photography, pinhole photography is from time to time a trend in artistic photography.

Related cameras, image forming devices, or developments from it include Franke's widefield pinhole camera, the pinspeck camera, and the pinhead mirror.

NASA (via the NAIC) has funded initial research into the New Worlds Imager project, which proposes to use a pinhole camera with a diameter of 10 m and focus length of 200,000 km to image earth sized planets in other star systems.

Source

This article comes from the English wikipedia, see http://en.wikipedia.org/wiki/Pinhole_camera.

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