Image stabilization
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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.
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[edit] Application in still photography
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 125 mm, 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 a camera uses. For example, many digital SLR cameras use an APS-C image sensor that multiplies the effective focal length of the lens by 1.5 or 1.6, depending on the camera. This value is referred to as the crop factor, field-of-view crop factor, focal-length multiplier, or format factor.
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 include a secondary panning mode or a more aggressive 'active mode', both described in greater detail below under optical image stabilization.
[edit] Techniques of image stabilization
[edit] Optical Image Stabilization
An Optical Image Stabilizer, often abbreviated as OIS or OS, 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 are other techniques described below. The primary reason for this to avoid building glass lenses into the camera body which would increase its size dramatically, and placing the IS inside the lens allows for the system to be fine-tuned for the characteristics of that particular lens. 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), 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. 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.[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 others 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. 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.[7] 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.
Image stabilization is also effective in reducing blur resulting from mirror-slap in newer versions of IS only;[citation needed] mirror lockup can be utilized to compensate instead when using an older IS system or none at all.
[edit] Moving the image sensor
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 [8]. 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 mounted. Konica Minolta used a technique called "anti-shake" now marketed as SteadyShot in the Sony α line and "shake reduction - SR" in the K10D and K100D lines by Pentax, which relies on a very precise angle speed sensor to detect camera motion.[9] Olympus introduced image stabilization with their E-510 D-SLR body, employing a system built around their Supersonic Wave Drive.[10] 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[11].
The advantage with moving the image sensor, instead of the lens, is that the image will be automatically stabilized regardless of what lens you are using. This means that buying another lens for your camera is cheaper, and the lenses weigh less.
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.
[edit] Digital image stabilization
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.
[edit] Stabilization filters
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.[citation needed] 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.[citation needed]
[edit] Orthogonal Transfer CCD
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.[12]
[edit] Stabilizing the camera body
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 for 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.[13]
[edit] References
- ^ Ken Rockwell, "Why IS and VR Matter"
- ^ What is Optical Image Stabilizer?, Technology FAQ, Canon Broadcast Equipment
- ^ Glossary : Optical : Image Stabilization, Vincent Bockaert, Digital Photography Review
- ^ Panasonic Mega OIS Explained
- ^ Vibration Reduction (VR) Technology
- ^ CameraHobby: Nikon AF-S VR 70-200mm f2.8 Review
- ^ Canon IS vs. Nikon VR
- ^ [1], Development of a Test Method for Image Stabilization Systems
- ^ Dynax 7D Anti-Shake Technology, Konica Minolta
- ^ Olympus Image Stabilization Technology
- ^ http://www.outdoorphotographer.com/content/2007/mar/image-stabilization3.shtml
- ^ Pan-STARRS Orthogonal Transfer CCD Camera Design, Gareth Wynn-Williams, Institute for Astronomy
- ^ Multimedia: Use Image Stabilization, Andy King, Web Site Optimization, 2004
