Camera shake is caused by a combination of the photographer’s hand movements or inability to keep the camera still, slow shutter speed, and long focal length. Camera shake results in a blurred image. The focal length of the lens, combined with a slow shutter speed, creates a situation in which the shutter speed is too slow to freeze the image before the camera moves significantly.
You can eliminate camera shake by using a tripod or by increasing the shutter speed to a value higher than the focal length. For example, if you’re shooting at a focal length equivalent to 100 mm, you should set your shutter speed to 1/100 of a second or faster. The digital image sensor will capture the image before the movement of the lens has time to register additional light information on the sensor.
Note: Some lenses have image stabilization features that allow the photographer to shoot at a shutter speed whose value is lower than the focal length of the lens.