Other Free Encyclopedias » Online Encyclopedia » Encyclopedia - Featured Articles » Contributed Topics from P-T » Talbot, William Henry Fox


film sensitive camera silver

Photography uses two basic principles, both long-known: firstly, a lens to form an image, as in the ‘camera obscura’ (a box with a lens in one end that forms an image on a screen at the other end for an artist to trace or copy); secondly, the sensitivity to light of some chemical compounds, mostly silver halides. Around 1800 , aided by his friend , tried to use these two principles to make stable images, but had little success. He used paper soaked in silver nitrate, but it was not sensitive enough to use in a camera, and his best results were shadow pictures of leaves and similar objects, obtained by placing them in contact with his paper and exposing to sunlight, when the uncovered areas blackened. He failed to find a way of ‘fixing’ the result, which blackened entirely on viewing in daylight.

In France, had more success using a pewter sheet covered in bitumen, although it was his partner who gained fame. Daguerre used a silvered copper sheet treated with iodine, which was sensitive enough to use in a camera. It was slow to respond and gave an image rich in detail but fragile and laterally reversed. It was also unique, in the sense that copies could not be made from it. Although used for some 15 years, the method was a dead end and is now extinct.

The scheme which led to modern photography and photomechanical printing was devised by and his friend in the late 1830s. Talbot used paper sensitized with silver salts, and found that the slight or invisible ‘latent’ image formed on it in the camera could be ‘developed’ by chemicals such as gallic acid. The result was fixed by soaking in ‘hypo’ solution, a method due to Herschel. The dried paper negative could then be oiled or waxed to made it transparent and used to make any number of positive prints (which would be the right way round) by contact printing on sensitive paper, which would be developed, fixed and washed as before. Herschel saw that a glass plate would be better in many ways than a paper base for the negative, but the film of photosensitive silver salts on glass was fragile and difficult to prepare, even when improved (as proposed by a cousin of Niépce) by the addition of egg albumen. The albumen was replaced by collodion by in 1851. Even with this improvement the business of making a photographic negative was burdensome, because the film was only sensitive if freshly prepared and still moist. It then had to be developed immediately, so a portable laboratory was needed. The photography of mountains, wars and tropical scenes on wet collodion plates required heroes for its achievement.

The next advance was due to R L Maddox (1816–1902), a doctor and an enthusiastic photographer whose asthma was irritated by the ether fumes from collodion (which is guncotton dissolved in ether and alcohol). After many trials he found that a suspension (‘emulsion’) of silver salts in gelatin would serve, and a film of this on a glass plate could be used dry. This made photography much easier for the amateur. The realization in the 1870s that storing and warming the emulsion, ‘ripening’ it, made it more photosensitive was important. It allowed exposure times of 1/25 s or less in place of seconds or minutes, and so moving subjects could be photographed. By the 1880s could use cameras to study the rapid animal movement of horses, men and women.

  Despite these advances, the sensitive film was still supported on glass, heavy and breakable, and cameras were usually wooden, large and tripod-mounted. Easy popular photography is due to George Eastman (1854–1932) of the USA, who made film-handling simple by mounting the sensitive gelatin emulsion on a strip of flexible plastic (originally celluloid), which could be protected from light with a length of black paper and then loaded into a lightweight camera as a rolled cartridge. From about 1900 this ‘Kodak’ system dominated popular photography. With , Eastman also devised the perforated plastic film that made cine-photography a practical success. From 1927 a track on one side of the perforated film met the need for sound to be linked with the moving image, ensuring a major place for the cinema in entertainment.

The key element in photography has always been the photosensitive silver component in the film, leading to images in which the dark areas are formed by small particles of metallic silver. Black-and-white photographs were the main outcome of both still and cine photography in its first century, but images in colour were always a target. Herschel and later had some early success in colour photography. Only after the Second World War did colour become popular, using ingenious thin sandwiches of coloured organic dyes linked with the sensitive silver layer and with development methods complex enough to be carried out normally in a commercial laboratory. The Polaroid ® camera due to uses rapid in-camera colour processes that are rather costly but provide ‘instant’ results.

Cameras have also increased greatly in sophistication. Modern lenses with several glass elements give good correction of optical defects while still admitting enough light for short exposures, and often allow a change of focal length (zoom designs) to vary the apparent perspective. Autofocus devices use infrared sensors to allow automatic adjustment of the lens-to-film distance, suitable for subjects at different distances. Photosensitive electronic devices measure light intensity and adjust lens aperture and/or shutter speed to ensure that an optimum level of light reaches the film.

The early photographers were as much concerned with contact photography as with camera work, and the main process used became Herschel’s cyanotype method, which gave the ‘blueprints’ used by in the 1840s and which dominated the production of engineer’s drawings in the 20th-century, until in the 1950s a very different method, ‘xerography’, based on electrostatic principles, was developed by .

The most recent advance in photography is based on a reusable sensitive surface and an electronic system and avoids ‘wet chemistry’. A CCD (charge-coupled device) whose surface is made of tiny light-sensitive pixels captures the image. Its output is handled digitally thereafter, and retained on a computer disc or card. The image can be manipulated by a PC in many ways: it can be scanned, colours changed, e-mailed, included in a web page, inkjet-printed, etc. Resolution is determined by the number of pixels in the CCD: 3 megapixels in a mid-price camera is common. These assets and its speed and availability, must lead to a dominant future for digital photography.

User Comments

Your email address will be altered so spam harvesting bots can't read it easily.
Hide my email completely instead?

Cancel or