1839 could be from 5 to 30 minutes. After the introduction of the Giroux daguerreotype camera, other manufacturers quickly produced improved variations. Charles Chevalier, who had earlier provided Niépce with lenses, created in 1841 a double-box camera using a half-sized plate for imaging. Chevalier's camera had a hinged bed, allowing for half of the bed to fold onto the back of the nested box. In addition to having increased portability, the camera had a faster lens, bringing exposure times down to 3 minutes, and a prism at the front of the lens, which allowed the image to be laterally correct. Another French design emerged in 1841, created by Marc Antoine Gaudin. The Nouvel Appareil Gaudin camera had a metal disc with three differently-sized holes mounted on the front of the lens. Rotating to a different hole effectively provided variable f-stops, allowing different amounts of light into the camera. Instead of using nested boxes to focus, the Gaudin camera used nested brass tubes. In Germany, Peter Friedrich Voigtländer designed an all-metal camera with a conical shape that produced circular pictures of about 3 inches in diameter. The distinguishing characteristic of the Voigtländer camera was its use of a lens designed by Joseph Petzval. The Petzval lens was nearly 30 times faster than any other lens of the period, and was the first to be made specifically for portraiture. Its design was the most widely used for portraits until Carl Zeiss introduced the anastigmat lens in 1889. Within a decade of being introduced in America, 3 general forms of camera were in popular use: the American- or chamfered-box camera, the Robert's-type camera or "Boston box", and the Lewis-type camera. The American-box camera had beveled edges at the front and rear, and an opening in the rear where the formed image could be viewed on ground glass. The top of the camera had hinged doors for placing photographic plates. Inside there was one available slot for distant objects, and another slot in the back for close-ups. The lens was focused either by sliding or with a rack and pinion mechanism. The Robert's-type cameras were similar to the American-box, except for having a knob-fronted worm gear on the front of the camera, which moved the back box for focusing. Many Robert's-type cameras allowed focusing directly on the lens mount. The third popular daguerreotype camera in America was the Lewis-type, introduced in 1851, which utilized