Image stabiliser; Image stabilisation
The term “Image stabilisation” represents a number of technologies for reducing blurring from camera movement during exposure. The image stabilisation mechanism compensates for movement in pan (lateral twisting or yawing movement) and tilt (vertical or pitching movement). When camera movement occurs image stabilisation technology stabilises the image projected on the sensor prior to the digital image sensor capturing the image data.
Typically optical stabilisation systems compensate for camera movement allowing for shutter speeds between two and four stops slower (eg. allows for exposures of four to sixteen times longer). In practical terms this means a hand held shot at 250th of a second shutter speed could be taken with 1/32nd or 1/15th of a second and still show similar image quality.
Motion blur can be created by moving the camera through a sweep of an arm or by other extreme movements. Image stabilisation does not prevent motion blur. Image stabilisation systems are designed to detect and compensate for the tiny motions and movements created by hand-holding a camera and lens. This allows hand holding for longer exposures.
Some image stabilisation systems have an additional feature. They compensate for movement in only the vertical plane. This allows the photographer to pan the camera to induce motion blur without the lens attempting to compensate. Consult your manual to see how to activate it if your system has this feature.
Stabilisation can occur in the lens. Sensors detect the camera movement and magnets are used to adjust a floating lens element to correct the optical path of light passing through the lens. The disadvantage of this technology is that every new lens has to have it’s own stabilisation mechanism and therefore makes the lens more expensive. However, in-lens stabilisation typically performs better than stabilisation in the camera body when telephoto or long focal length lenses are in use. This is because the telephoto effect exaggerates the effect of vibration. Under these circumstances correction in the lens is easier and can compensate.
Some cameras use stabilisation motors to adjust the position of the image sensor itself to compensate for the camera movement. This “sensor shift” technology is in the camera body. The cost of the mechanism is only paid for in the purchase of the camera body. Lenses are therefore cheaper as they do not each have inbuilt stabilisation. Lenses are also lighter and less complex. The sensor shift technologies have a larger image circle projected on to them by the lens. This is because the sensor needs to move around to compensate for the vibration. If the vibration errors are not corrected in the lens then the variation at the sensor can get quite large. For long lenses the required movement of the sensor can get extreme. Sensor shift technologies are therefore said to be less effective with long lenses and significant vibration or movement. They are limited by the maximum effective movement allowed by the sensor.
There are a number of external stabilisation methods including gyroscopic compensations systems, cushion systems and motorised systems. These tend to be used for specialist purposes and more often for video photography than stills photography. The most common external system for stabilising cameras is the tripod. In fact tripods provide the best stabilisation for all situations.
Many sources say the best practice is to turn off image stabilisation systems when using a tripod. The explanation for this is that the stabilisation mechanism itself creates vibrations. This vibration is magnified by the tripod creating a feedback vibration which softens the image.
Some more recent image stabilisation systems are designed to detect the lack of vibration when on a tripod and turn the stabilisation system off automatically. However, this is unreliable. Vibrations can be induced in a tripod by ground vibration, wind and the camera itself. These detection systems are effective when external vibration definitely does not affect the tripod. However, sometimes these vibrations are not detectable to us and do not affect the exposure but are detected by the automatic detection system. If these tiny vibrations are detected then the image stabilisation system will turn itself back on. The initialisation process and the operation of the system does affect the tripod and may soften the shot. These systems are improving. However, manufacturers are not at present (2013) confident enough to make strong claims about the effectiveness of these detection systems under all circumstances. Photographers should research the effectiveness of the system they own and test manufacturer statements about on-tripod detection of vibration. Alternatively, the best advice may be to turn off stabilisation systems when on a tripod.