expressionistic acting styles, such as Metropolis, were still being released. Greta Garbo, who made her debut in 1926, would become known for her naturalistic acting. According to Anton Kaes, a silent film scholar from the University of California, Berkeley, American silent cinema began to see a shift in acting techniques between 1913 and 1921, influenced by techniques found in German silent film. This is mainly attributed to the influx of emigrants from the Weimar Republic, "including film directors, producers, cameramen, lighting and stage technicians, as well as actors and actresses". Projection speed Until the standardization of the projection speed of 24 frames per second (fps) for sound films between 1926 and 1930, silent films were shot at variable speeds (or "frame rates") anywhere from 12 to 40 fps, depending on the year and studio. "Standard silent film speed" is often said to be 16 fps as a result of the Lumière brothers' Cinématographe, but industry practice varied considerably; there was no actual standard. William Kennedy Laury Dickson, an Edison employee, settled on the astonishingly fast 40 frames per second. Additionally, cameramen of the era insisted that their cranking technique was exactly 16 fps, but modern examination of the films shows this to be in error, and that they often cranked faster. Unless carefully shown at their intended speeds silent films can appear unnaturally fast or slow. However, some scenes were intentionally undercranked during shooting to accelerate the action—particularly for comedies and action films. Slow projection of a cellulose nitrate base film carried a risk of fire, as each frame was exposed for a longer time to the intense heat of the projection lamp; but there were other reasons to project a film at a greater pace. Often projectionists received general instructions from the distributors on the musical director's cue sheet as to how fast particular reels or scenes should be projected. In rare instances, usually for larger productions, cue sheets produced specifically for the projectionist provided a detailed guide to presenting the film. Theaters also—to maximize profit—sometimes varied projection speeds depending on the time of day or popularity of a film, or to fit a film into a prescribed time slot. All motion-picture film projectors require a moving shutter to block the light whilst the film is moving, otherwise the image is smeared in the direction of the movement. However this shutter causes the image to flicker, and images with