Image stabilization (IS) is a family of techniques used to reduce blurring associated with the motion of a camera during exposure. Specifically, it compensates for pan and tilt (angular movement, equivalent to yaw and pitch) of a camera or other imaging device. It is used in image-stabilized binoculars, still and video cameras, 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 problem of lens-shake is added to by variations in the atmosphere over time, which cause the apparent positions of objects to change.
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In photography, image stabilization can often permit the use of shutter speeds 2–4 stops slower (exposures 4–16 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 35mm equivalent focal length of the lens. For example, at a focal length of 125 mm on a 35mm camera, vibration or camera shake could 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 a camera uses. For example, many digital SLR cameras use an image sensor that is 2/3, 5/8, or 1/2 the size of a 35mm film frame. This means that the 35 mm frame is 1.5, 1.6, or 2 times the size of the digital sensor. The latter values are referred to as the crop factor, field-of-view crop factor, focal-length multiplier, or format factor. On a 2x crop factor camera, for instance, a 50mm lens produces the same field of view as a 100mm lens used on a 35mm film camera, and can typically be handheld at 1/100 of a second.
However, image stabilization 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 and camera bodies include a secondary panning mode or a more aggressive 'active mode', both described in greater detail below under optical image stabilization.
Image-stabilization features can also be a benefit in astrophotography, when the camera is technically- but not effectively- fixed in place. The Pentax K-5 and K-r can use their sensor-shift capability to reduce star trails in reasonable exposure times, when equipped with a GPS accessory for position data. In effect, the stabilization compensates for the Earth's motion, not the camera's.[2]
There are two types of implementation - lens-based, or body-based stabilization. These refer to where the stabilizing system is located. Both have their advantages and disadvantages.[3]
An optical image stabilizer, often abbreviated OIS, IS, or OS, is a mechanism used in a still camera or video camera that stabilizes the recorded image by varying the optical path to the sensor. This technology is implemented in the lens itself, or by moving the sensor as the final element in the optical path. The key element of all optical stabilization systems is that they stabilize the image projected on the sensor before the sensor converts the image into digital information.
Different companies have different names for the OIS technology; for example: Image Stabilization (IS - Canon, the first to produce an OIS lens), Vibration Reduction (VR - Nikon), Optical SteadyShot (Sony Cyber-Shot), MegaOIS (Panasonic and Leica), Super Steady Shot (SSS - Sony), Optical Stabilization (OS - Sigma), Vibration Compensation (VC - Tamron) and Shake Reduction (SR - Pentax).
In Nikon and Canon's implementation, it works by using a floating lens element that is moved orthogonally to the optical axis of the lens using electromagnets.[4] Vibration is detected using two piezoelectric angular velocity sensors (often called gyroscopic sensors), one to detect horizontal movement and the other to detect vertical movement.[5] As a result, this kind of image stabilizer only corrects for pitch and yaw axis rotations,[6][7] 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, or it can be 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'.[8] However, Active Mode, when used under normal shooting conditions, can result in poorer results than the 'Normal Mode'.[9] This is because active mode is optimized for reducing higher angular velocity movements (typically when shooting from a heavily moving platform using faster shutter speeds), where normal mode tries to reduce lower angular velocity movements over a larger ampliture and timeframe (typically body and hand movement when standing on a stationary or slowly moving platform while using slower shutter speeds).
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. Many modern image stabilization lenses (notably Canon's more recent IS lenses) are able to auto-detect that they are tripod-mounted (as a result of extremely low vibration readings) and disable IS automatically to prevent this and any consequent image quality reduction.[10] The system also draws power from the battery, so de-activating it when it is not needed will extend the time before a recharge is required.
One of the main disadvantages about lens-based image stabilization is the higher price tag that comes with it; image stabilisation has to be paid for each lens anew. Also, not every lens is available as an image-stabilised variant. This is often the case for fast primes and wide-angle lenses. While the most obvious advantage for image stabilisation lies with longer focal lengths, even normal and wide-angle lenses benefit from it in low-light applications. Furthermore, because light passing through the lens is shifted from its true optical path when it projects out the rear element onto the sensor, poor 'bokeh' can result.[11] However, this could be considered and compensated during the design stage of the lens. Lens based stabilization also has advantages over in-body stabilization. In low-light or low-contrast situations, the autofocus system (which has no stabilized sensors) is able to work more accurately when the image coming from the lens is already stabilized. In cameras with optical viewfinders, the image seen by the photographer through the stabilized lens (as opposed to in-body stabilization) reveals more detail because of its stability, and it also makes correct framing easier. This is especially the case with longer telephoto lenses.
The sensor capturing the image can be moved in such a way as to counteract the motion of the camera, a technology often referred to as mechanical image stabilization. When the camera rotates, causing angular error, gyroscopes encode information to the actuator that moves the sensor.[12] The sensor is moved to maintain the projection of the image onto the image plane, which is a function of the focal length of the lens being used; modern cameras can acquire focal length information from the lens. Konica Minolta used a technique called "anti-shake" now marketed as SteadyShot in the Sony α line and "shake reduction - SR" in the K-5, K-7, K10D, K20D, K100D, K200D, K-m (K-2000) and K-x lines by Pentax, which relies on a very precise angular rate sensor to detect camera motion.[13] Olympus introduced image stabilization with their E-510 D-SLR body, employing a system built around their Supersonic Wave Drive.[14] Other manufacturers use DSPs to analyze the image on the fly and then move the sensor appropriately. Sensor shifting is also used in some cameras by Fujifilm, Pentax, Samsung, Casio Exilim and Ricoh Caplio.[15]
The advantage with moving the image sensor, instead of the lens, is that the image will be stabilized regardless of what lens is being used. This allows the stabilization to work with any lens the photographer chooses and reduces the weight and complexity of the lenses. Further, when sensor-based image stabilization technology improves, it only requires replacing the camera to take advantage of the improvements, which is typically far less expensive than replacing all existing lenses if relying on lens-based image stabilization. Some sensor-based image stabilization implementations are capable of correcting camera roll rotation, a motion that is easily excited by pressing the shutter button. No lens-based system can address this potential source of image blur. A by-product of available "roll" compensation is that the camera can automatically correct for tilted horizons in the optical domain, provided it is equipped with an electronic spirit level, such as the Pentax K-7/K-5 cameras.
One of the primary disadvantages of moving the image sensor itself is that the image projected to the viewfinder is not stabilized. However, this is not an issue on cameras that use an electronic viewfinder (EVF), since the image projected on that viewfinder is taken from the image sensor itself. Similarly, the image projected to a phase-detection autofocus system, if used, is not stabilized.
In-body image stabilization requires the lens to have a larger output image circle because the sensor is moved during exposure and thus uses a larger part of the image. Compared to lens movements in optical image stabilisation systems the sensor movements are quite large, so the effectiveness is limited by the maximum range of sensor movement, where a typical modern optically stabilized lens has greater freedom. The required sensor movement (both speed and range) increase with the focal length of the lens being used, making sensor-shift technology less suited for very long telephoto lenses, especially when using slower shutter speeds because the available motion range of the sensor quickly becomes insufficient to cope with the increasing image displacement.
Real-time 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. This technique reduces distracting vibrations from videos or improves still image quality by allowing one to increase the exposure time without blurring the image. This technique does not affect the noise level of the image, except in the extreme borders when the image is extrapolated.
Unfortunately, many still camera manufacturers market their cameras as having "digital image stabilization", when they really only have a mode with a sub-optimal image exposure time, resulting in pictures with less motion blur, but more noise.[16]
The main disadvantage with this solution is that motion blur from shake under medium to low light conditions will be impossible to correct by the post processing images stabilization system, so in real life this will only work well for short exposure times (daylight).
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.[17] 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 spatial or temporal extrapolation.[18]
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.[19]
A technique that requires no additional capabilities of any camera body–lens combination consists of stabilizing the entire camera body externally rather than using an internal method. This is achieved by attaching a gyroscope to the camera body, usually utilizing the camera's built-in tripod mount. This allows the external gyro to stabilize the camera, and is typically employed in photography from a moving vehicle, when a lens or camera offering another type of image stabilization is not available.[20]
Another technique for stabilizing a video or motion picture camera body is the Steadicam system which isolates the camera from the operator's body using a harness and a camera boom with a counterweight.[21]
In many animals, including human beings, the inner ear functions as the biological analogue of an accelerometer in camera image stabilization systems, to stabilize the image by moving the eyes. When a rotation of the head is detected, an inhibitory signal is sent to the extraocular muscles on one side and an excitatory signal to the muscles on the other side. The result is a compensatory movement of the eyes. Typically eye movements lag the head movements by less than 10 ms.