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Image stabilization (IS) is a family of techniques to increase the stability of an image. It is used in image-stabilized binoculars, photography, videography, and astronomical telescopes. With still cameras, camera shake is particularly problematic at slow shutter speeds or with long focal length (telephoto) lenses. With video cameras, camera shake causes visible frame-to-frame jitter in the recorded video. In astronomy, the problems of lens-shake are compounded by variations in the atmosphere over time, which cause the apparent position of objects to move.
In photography, image stabilization can often permit the use of shutter speeds 3-4 stops slower (exposures 8-16 times longer), although even slower effective speeds have been reported.
The rule of thumb to determine the slowest shutter speed possible for hand-holding without noticeable blur due to camera shake is to take the reciprocal of the effective focal length of the lens. For example, at a focal length of 125mm, vibration or camera shake would affect sharpness if the shutter speed was slower than 1/125 second. As a result of the 3-4 stops slower shutter speeds allowed by IS, an image taken at 1/125 second speed with an ordinary lens could be taken at 1/15 or 1/8 second with an IS-equipped lens and produce almost the same quality. The sharpness obtainable at a given speed can increase dramatically.[1] When calculating the effective focal length, it is important to take into account the image format your camera uses. For example, many digital SLR cameras use an APS-C image sensor that effectively multiplies the focal length of the lens by 1.5 or 1.6, depending on the camera; check your camera's specifications to learn this number, sometimes referred to as the field of view crop factor.
It is important to note, however, that the use of image stabilization to permit shutter speeds slower than the general rule recommends (shutter speed equal to 1/focal length) does not prevent motion blur caused by the movement of the subject or by extreme movements of the camera. Image stabilization is only designed for and capable of reducing blur that results from normal, minute shaking of a lens due to hand-held shooting. Some lenses include a secondary panning mode or a more aggressive 'active mode', both described in greater detail below under optical image stabilization.
An Optical Image Stabilizer, often abbreviated as OIS, is a mechanism used in a digital still camera or video camera that stabilizes the recorded image by varying the optical path to the sensor.[2] This technology is implemented in the lens itself, rather than in the camera as other techniques described below do; all of the OIS functions occur within the lens itself. Different companies have different names for this technology; for example, Canon, the pioneer of the technology and the first to produce an OIS lens, calls it Image Stabilization (IS), Nikon calls it Vibration Reduction (VR), and Panasonic calls it MegaOIS.
In Canon's implementation, it works by using a floating lens element that is moved orthogonally to the optical axis of the lens, using electromagnets. The vibration signal which is compensated for by the stabilizing lens element is typically acquired using two piezoelectric angular velocity sensors (often called gyroscopic sensors), one to detect horizontal movement and the other to detect vertical movement[3]. As a result, this kind of image stabilizer only corrects for pitch and yaw axis rotations,[4] and cannot correct for rotation around the optical axis. Some lenses have a secondary mode that counteracts vertical camera shake only. This mode is useful when using a panning technique, and switching into this mode depends on the lens, sometimes it is done by using a switch on the lens and on other lenses it is automatic.
Some of Nikon's more recent VR-enabled lenses offer an 'Active Mode' that is intended to be used when shooting from a moving vehicle, such as a car or boat, and should correct for larger shakes than the 'Normal Mode'.[5] However, Active Mode, when used under normal shooting conditions, can result in poorer results than the 'Normal Mode'.[6]
Most manufacturers suggest that the IS feature of a lens be turned off when the lens is mounted on a tripod, as it can cause erratic results and is generally unnecessary. Image stabilization is not effective in reducing blur resulting from mirror-slap[7], which should be prevented using mirror lockup instead. Tests have shown that turning IS off when the camera is mounted on a tripod actually produces sharper images. However, some lenses (notably Canon's more recent IS lens) are able to auto-detect that they are tripod-mounted (as a result of extremely low vibration readings) and disable IS themselves to prevent erratic behavior and the above-mentioned reduced image quality.[7]
The sensor capturing the image may be moved in such a way as to counteract the motion of the camera. Konica Minolta used a technique called "anti-shake" now marketed as SteadyShot in the Sony α line, which relies on a very precise angle speed sensor to detect camera motion.[8] Other manufacturers use DSPs to analyze the image on the fly and then move the sensor appropriately.
Digital image stabilization is used in some video cameras. This technique shifts the electronic image from frame to frame of video, enough to counteract the motion. It uses pixels outside the border of the visible frame to provide a buffer for the motion.
Many Non-linear editing systems use stabilization filters that can correct a non-stabilized image by tracking the movement of pixels in the image and correcting the image by moving the frame. The process is similar to Digital image stabilization but since there is no "larger" image to work with the filter either crops the image down to hide the motion of the frame or attempts to recreate the lost image at the edge through extrapolation.
Used in astronomy, an orthogonal transfer CCD (OTCCD) actually shifts the image within the CCD itself while the image is being captured, based on analysis of the apparent motion of bright stars. This is a rare example of digital stabilization for still pictures. An example of this is in the upcoming gigapixel telescope Pan-STARRS being constructed in Hawaii.[9]
A technique that requires no additional capabilities of any camera body or lens combination is the technique of stabilizing the entire camera body externally rather than using and internal method. This is achieved by attaching a gyroscope to the camera body, usually to the camera's built-in tripod mount. This allows for an external gyro to stabilize the camera and is typically employed in photography in a moving vehicle when a lens or camera offering another type of image stabilization is not available.[10]