Camera shake is a major cause of blurred image especially with telephoto lenses. Normally, a shutter speed at least as fast as the reciprocal of the lens focal length, (1/focal length) sec., can prevent a blurred image due to camera shake. However, under low-light conditions or with slow film, a slower shutter speed will be required, resulting in image blur for handheld shots. Canon started development of IS (Image Stabilizer) technology in 1980s and introduced EF75-300mm f/4-5.6 IS USM in 1995, the world's first interchangeable lens for 35mm SLR with a built-in image stabilizer. The vibration detecting gyro sensor detects the level of camera shake and the actuator moves a part of the optical system (IS lens group) vertically to the optical axis depending on the degree of camera shake, to stabilize the image on the film plane. Normal image stabilizing is suitable for shooting of stationary subject. However, EF300mm f/4 L IS USM has "IS Mode 2" to stabilize finder images at panning shots of moving subjects.