The daguerreotype, however, indicated some shortcomings. The central problem was that they could be reproduced only by photographing the original image, because each picture was an original and the metal process using no negatives. In addition, the earliest daguerreotypes had exposure times ranging from three to fifteen minutes, which is why they were impractical for portraiture. Unlike previous drawings could represent no movement. The models had to hold their poses for a long time very quiet. Therefore, the standard pornographic picture changed by several people for sexual acts to a single woman who makes her genitals on display. Since one picture could cost a week's salary, the audience consisted mainly of those nude photographs of artists and the upper class. In the 1840s it was cheaper to hire a prostitute to experience sex, than to have a picture of it. 1838, the stereoscope was invented, which was popular in the erotic images including daguerreotypes. The technology allowed a three-dimensional view, which was well suited to erotic images. Although thousands of erotic daguerreotypes have emerged, survived only about 800 copies. Given its uniqueness and cost they were once the plaything of rich men. Because they are so rare, they fetch prices of more than £ 10,000.