Daguerreotype - Reduction of Exposure Time

Reduction of Exposure Time

The very first daguerreotypes used Chevalier lenses that were "slow", and the light sensitive material was silver iodide made by fuming the plate with iodine vapor. This meant that the exposure in the camera was too long to conveniently take portraits commercially, and the first subjects taken were immobile subjects such as street scenes, still life architectural studies, reclining nudes etc.

Two changes were introduced that shortened the exposure times: one was fitting lenses with a larger diameter to the camera, and the other was a modification to the chemistry used.

When Petzval lenses were introduced in 1841, with a larger effective aperture and the plate was sensitized not only with iodine but also with bromine and chlorine and forming light sensitive crystals of silver iodide, silver bromide and/or silver chloride that are more light sensitive than silver iodide the exposures were reduced (the lens remaining uncapped for a shorter time) so that commercial portraits could be taken. Increased speed was achieved using the same chemistry in the later silver processes that followed. Usually, it was arranged so that the sitters leaned their elbows on a support such as a posing table whose height could be adjusted or else head rests were used that did not show in the picture and this led to most daguerreotype portraits having stiff, lifeless poses. There were exceptions with lively expressions full of character by photographers who saw the potential of the new medium, and these are represented in museum collections and are the most sought after by private collectors today.Daguerreotypes were mounted in cases under glass with a cover, or else in a frame that could be hung on a wall. They were usually sealed with tape to reduce oxidization and tarnishing of the plate as well as mechanical damage from being touched.

Photographic processes that were invented soon after: ambrotypes and tintypes were mounted in similar cases, but were made by the later wet plate process using collodion on glass or on a bitumen coated iron plate. These can be distinguished from daguerreotypes by the image quality. The polished silver surface of a daguerreotype gives a feeling of presence where the image appears to be floating in space.

The process was developed by Louis Daguerre together with Joseph Nicéphore Niépce. Niépce had produced the first photographic image in the camera obscura with an eight hour exposure using bitumen of Judea on a pewter plate developing it in lavender oil, a process he called heliography.

The bitumen hardened where light had affected it and the non exposed portions were washed away.

Thomas Wedgwood had made outlines and silhouettes of shapes as well as using a glass negative by painting on glass to produce images on white leather using silver chemistry, but he was not successful in producing an image in the camera obscura and neither did he have a method to fix the image by dissolving out the unexposed silver salts. His images had to be viewed in a dimly lit room, and they gradually blackened entirely with exposure to light.

The image in a daguerreotype is often described as being formed by the amalgam, or alloy, of mercury and silver because mercury vapor from a pool of heated mercury is used to develop the plate; but using the Becquerel process (using a red filter and two-and-a-half stops extra exposure) daguerreotypes can be produced without mercury, and chemical analysis shows that there is no mercury in the final image with the Bequerel process. This leads to questioning the theory that the image is formed of amalgam with mercury development.

Exposure times were later reduced by sensitizing the plate with other silver halides: silver bromide and silver chloride, and by replacing the Chevalier lenses with much larger, faster lenses designed by Joseph Petzval. A reduction in camera size and the size of the image will always result in more light reaching the image plane and consequently reduced exposures. This principle was used in Voigtländer's all metal Daguerrotype camera where a smaller image resulted in reducing the exposure time to two seconds.The camera did not catch on and was not a marketing success.

Although daguerreotypes are unique images, they could be copied by redaguerreotyping the original. although this was not easy to do.