Image stabilization or IS is a Canon technology that optically corrects for camera motion when you take a photo. Since camera motion - caused by handholding the camera, for example - can result in blurring of the image at slower shutter speeds, IS can result in sharper photographs when fast shutter speeds are not possible.

IS is a fairly complex technology involving motion sensors, microcomputer chips and small motors to move key lens elements. There is, therefore, a price premium for IS-capable lenses. But they can be very convenient - when handholding a camera you can easily gain a stop or two over using a non-IS lens.

However, remember that IS does not increase the maximum aperture of the lens or anything. An IS lens with a maximum aperture of 3.5 still has a maximum aperture of 3.5. IS simply lets you use a slower shutter speed than would otherwise be possible when you’re handholding the camera, by compensating for camera motion. So you won’t necessarily be able to get that narrow depth of field that you could with a faster lens - which could be a drawback or a benefit depending on your point of view.

IS has a few other drawbacks over faster lenses as well. Earlier IS lenses tended not to perform very well when mounted on a tripod when the IS mechanism was engaged. Consumer IS lenses also do not work very well when panning (tracking a moving object), though pro IS lenses do. IS does not help you if the subject is moving - it compensates only for camera motion. IS doesn’t help freeze subject motion and in fact will probably make things worse by letting you use a much slower shutter speed than a fast lens. Some people find the slight swimming motion in the viewfinder when using IS a bit dizzying and IS uses a little more battery power than no stabilization at all. Finally, some earlier film EOS cameras are not entirely compatible with IS lenses and have minor inconveniences, such as viewfinder shake once a photo has been taken (though this does not affect the picture quality).

Nonetheless, these drawbacks aside, most people find IS quite valuable, particularly on long telephoto lenses.

Canon were the first company to include image stabilization technology in SLR lenses, though Nikon actually pioneered the field with a stabilized-lens point and shoot (the Zoom-Touch 105 VR) in 1994. Today Nikon sell a range of VR (“vibration reduction”) SLR lenses; the main difference being Nikon sell mainly to the high end lens market where Canon sell a variety of IS lenses covering the mid to high end markets. Sigma have also released a number of image-stabilized lenses. Canon IS technology is built into the lens, like the Mega Optical Image Stabilizer (Mega OIS) system used by Panasonic. These both differ from the Minolta-developed Super SteadyShot stabilization technology used by Sony, which is built into the camera body. Building anti-vibration technology into the camera has the advantage of making the feature available to any lens attached to a supported camera, but it has the disadvantage of not being tailored to each focal length range.