Vibration reduction (VR) systems are designed to help reduce the effects of camera shake during exposure. It is important to note the difference between subject movement and camera movement. Movement of the subject can be frozen by use of a sufficiently fast shutter speed but is not affected by VR systems. The effects of camera movement are also reduced by using fast shutter speeds, but in situations where this is not possible a VR system can make a significant difference to the sharpness of an image. Manufacturers claim potential improvements in sharpness that are the equivalent of using an exposure between two and four stops (typically three stops or 8 x) faster than that selected. For example, when using a hand-held 300mm lens with a shutter speed of 1/250 second, one might expect the image to exhibit the first signs of camera shake. When using a similar lens with a VR system, a photographer could expect to continue obtaining sharp images with shutter speeds as low as 1/60 second or 1/30 second.
There are basically two types of stabilization or vibration reduction system available. Some are built in the the camera body and others are fitted within particular lenses. Both systems produce worthwhile improvements in image sharpness when working with slow shutter speeds. Some systems can also cope with panning, where a camera is deliberately swung to track a moving subject. In such circumstances, a some VR systems reduce the effect of camera movement in the plane perpendicular to the path of the camera's panning motion.
It is important to realize that VR systems are not a guarantee that camera movement cannot ruin an image. In practice it is more a case of reducing the risk of camera movement. When working with a 300mm lens at 1/250 second there is a risk of camera shake being evident in the image. With a VR system active, that risk is substantially reduced and may be almost eliminated. With a 300mm lens used with a shutter speed of 1/60 second the risk of blurring due to camera shake is very high - perhaps 90%. However, with an active VR system the risk might be reduced to 25%. Even when using the best VR system available it is still possible to ruin images due to camera shake.
VR systems work by continuously detecting camera shake and making extremely fast changes to the position of various lens elements to counter its effects. Data from angular velocity sensors is processed almost instantaneously, and specialized motors in two planes are used to move the lens elements in the appropriate direction. VR systems typically take no more than one millisecond (1/1000 second) to complete such adjustments. One motor corrects for horizontal movement and another for movement in the vertical plane. When panning, firmware in the system detects the sustained movement in a particular direction and inhibits any attempt by the system to correct for it. However, compensation for movement in the perpendicular plane can still be made.