artist's studies. Many of the surviving examples of daguerreotypes are clearly not in this genre but have a sensuality that clearly implies they were designed as erotic or pornographic images". The daguerreotypes were not without drawbacks, however. The main difficulty was that they could only be reproduced by photographing the original picture since each image was an original and the all-metal process does not use negatives. In addition, the earliest daguerreotypes had exposure times ranging from three to fifteen minutes, making them somewhat impractical for portraiture. Unlike earlier drawings, action could not be shown. The poses that the models struck had to be held very still for a long time. Another limitation was the monochrome image that the technology could produce. Because of this, the standard pornographic image shifted from one of two or more people engaged in sex acts to a solitary woman exposing her genitals. The cost of the process also limited the spread of the technology. Since one picture could cost a week's salary, the audience for nudes mostly consisted of artists and the upper echelon of society. Stereoscopy was invented in 1838 and became extremely popular for daguerreotypes, including the erotic images. This technology produced a type of three dimensional view that suited erotic images quite well. Although thousands of erotic daguerreotypes were created, only around 800 are known to survive; however, their uniqueness and expense meant that they were once the toys of rich men. Due to their rarity, the works can sell for more than £GB 10,000. The calotype process In 1841, William Fox Talbot patented the calotype process, the first negative-positive process, making possible multiple copies. This invention permitted an almost limitless number of prints to be produced from a glass negative. The technology also reduced the exposure time and made possible a true mass market for low cost commercial photography. The technology was immediately employed to reproduce nude portraits, classified by the standards of the time as pornographic. Paris soon became the centre of this trade. In 1848 only thirteen photography studios existed in Paris; by 1860, there were over 400. Most of them made income from the sale of illicit nude images to the masses who could now afford it. The pictures were also sold near train stations, by traveling salesmen and women in the streets who hid them under their dresses. They were often produced in sets (of four, eight