In the mid-1820s, prior to his association with Daguerre,
Niépce used a coating of
bitumen of Judea to make the first permanent camera photographs. The bitumen was hardened where it was exposed to light and the unhardened portion was then removed with a solvent. A camera exposure lasting for hours or days was required. Niépce and Daguerre later refined this process, but unacceptably long exposures were still needed. After the death of Niépce in 1833, Daguerre concentrated his attention on the light-sensitive properties of silver salts, which had previously been demonstrated by
Johann Heinrich Schultz and others. For the process which was eventually named the
daguerreotype, he exposed a thin silver-plated copper sheet to the vapour given off by
iodine crystals, producing a coating of light-sensitive
silver iodide on the surface. The plate was then exposed in the camera. Initially, this process, too, required a very long exposure to produce a distinct image, but Daguerre made the crucial discovery that an invisibly faint "latent" image created by a much shorter exposure could be chemically "developed" into a visible image. Upon seeing the image, the contents of which are unknown, Daguerre said, "I have seized the light – I have arrested its flight!" '', taken by Daguerre in 1838 in Paris, includes the earliest known verified photograph of a person. The image shows a busy street, but because the exposure had to continue for four to five minutes the moving traffic is not visible. At the lower right, however, a man apparently having his boots polished, and the
bootblack polishing them, were motionless enough for their images to be captured. , where Daguerre took pictures in 1837. If correct, this would be the oldest surviving portrait photograph of a human being. The latent image on a daguerreotype plate was developed by subjecting it to the vapour given off by
mercury heated to 75 °C. The resulting visible image was then "fixed" (made insensitive to further exposure to light) by removing the unaffected silver iodide with concentrated and heated
salt water. Later, a solution of the more effective "hypo" (hyposulphite of soda, now known as
sodium thiosulfate) was used instead. The resultant plate produced an exact reproduction of the scene. The image was laterally reversed—as images in mirrors are—unless a mirror or inverting
prism was used during exposure to flip the image. To be seen optimally, the image had to be lit at a certain angle and viewed so that the smooth parts of its mirror-like surface, which represented the darkest parts of the image, reflected something dark or dimly lit. The surface was subject to tarnishing by prolonged exposure to the air and was so soft that it could be marred by the slightest friction, so a daguerreotype was almost always sealed under glass before being framed (as was commonly done in France) or mounted in a small folding case (as was normal in the UK and US). Daguerreotypes were usually
portraits; the rarer landscape views and other unusual subjects are now much sought after by collectors and sell for much higher prices than ordinary portraits. At the time of its introduction, the process required exposures lasting ten minutes or more for brightly sunlit subjects, so portraiture was an impractical ordeal.
Samuel Morse was astonished to learn that daguerreotypes of the streets of Paris did not show any people, horses or vehicles, until he realized that due to the
long exposure times all moving objects became invisible. Within a few years, exposures had been reduced to as little as a few seconds by the use of additional sensitizing chemicals and "faster"
lenses such as
Petzval's portrait lens, the first mathematically calculated lens. The daguerreotype was the
Polaroid film of its day: it produced a unique image which could only be duplicated by using a camera to photograph the original. Despite this drawback, millions of daguerreotypes were produced. The paper-based
calotype process, introduced by
Henry Fox Talbot in 1841, allowed the production of an unlimited number of copies by simple
contact printing, but it had its own shortcomings—the grain of the paper was obtrusively visible in the image, and the extremely fine detail of which the daguerreotype was capable was not possible. The introduction of the
wet collodion process in the early 1850s provided the basis for a negative-positive print-making process not subject to these limitations, although it, like the daguerreotype, was initially used to produce one-of-a-kind images—
ambrotypes on glass and
tintypes on black-lacquered iron sheets—rather than prints on paper. These new types of images were much less expensive than daguerreotypes, and they were easier to view. By 1860 few photographers were still using Daguerre's process. The same small ornate cases commonly used to house daguerreotypes were also used for images produced by the later and very different
ambrotype and
tintype processes, and the images originally in them were sometimes later discarded so that they could be used to display photographic paper prints. It is now a very common error for any image in such a case to be described as "a daguerreotype". A true daguerreotype is always an image on a highly polished silver surface, usually under protective glass. If it is viewed while a brightly lit sheet of white paper is held so as to be seen reflected in its mirror-like metal surface, the daguerreotype image will appear as a relatively faint
negative—its dark and light areas reversed—instead of a normal positive. Other types of photographic images are almost never on polished metal and do not exhibit this peculiar characteristic of appearing positive or negative depending on the lighting and reflections. == Competition with Talbot ==