And which one is more effective?
In lens stabilisation contains a servo activated rear element which acts to move the image projected by the lens in order to cancel out the camera shake.
In body stabilisation moves the sensor in order to counter camera shake.
No method is clearly better, and discussion tends to turn into a brand war as Canon and Nikon don't offer (and are not likely to offer) a body based solution, and therefore must adopt/reinforce the view than in-lens is better.
In lens stabilisation:
Can be tuned to the requirements of a specific lens. Telephoto lenses are likely to show a different pattern of blur (more linear, over a shorter time period) than wider angles, which have to stabilise the image for longer, where an oscillatory motion may be present.
The in-lens method stabilises what you see in the viewfinder for easier composition.
You can get hybrid systems that counter both angular and translational movement, which is important when the distance the subject is small. Currently the only exmaple is Canon's 100mm f/2.8L macro.
Works with all bodies, including film / older digital bodies, however most people have more lenses than bodies!
In body stabilisation
Provides stabilisation for every lens you mount, even old MF designs. Since stabilised lenses are almost always more expensive and stabilisation only exists on certain models this is a considerable plus.
No extra elements in the optical path to potentially flare or disperse light.
Can correct rotational movement and can automatically level the horizon.
Astronomical tracking mount like features are available on some models (such as the Pentax K-5, thanks John).
I've seen some tests that indicate the in lens stabilisation can perform better, however you do have to pay for it in every lens, see Image Stabilization Testing on SLRgear. It makes sense to me than in lens could perform slightly better as you're comparing a system designed for a specific focal length to a system that has to try and work in every situation. There's no reason in principle you couldn't have in-body stabilisation and then turn it off and use in-lens for some lenses for maximum performance.
Advantages of having stabilisation in the camera:
- Works for all lenses
- The lenses gets cheaper
Advantages of having stabilisation in the lens:
- The camera gets cheaper and can be built smaller
- The stabilisation can be specially made to fit the lens characteristics
- Works also with older camera models
Lenses are generally the most expensive part of photography equipment and likely to be kept longer than bodies. Assuming that mounts don't change it may be more cost effective to use cameras with in-body stabilization and take advantage of the certain technological improvements in the body which will occur every few years while keeping an investment in top quality lenses valid. Quality lenses are rarely improved upon quickly.
In-lens stabilisation uses a floating element in the lens that moves to compensate for movement. In-camera stabilisation moves the sensor to compensate instead.
While there is little really noticeable difference in image quality between the two, in-camera stabilisation has the advantages of:
- Being lens independent, so you don't need to buy several lenses with IS
- Making lens less complex, therefore cheaper and lighter
- Some in-camera systems can compensate for rotational movement, i.e. turning the camera, which can occur when you press the shutter button.
As innovations are made into image stabilisation, there is the possibility of including it in newer lenses, which can then be an advantage to pre-existing camera owners; rather than having to invest in a new camera body.
There are some corrections that will be virtually impossible to do with in-lens stabilisation, that could be achieved with in-camera stabilisation, such as correcting rotations.
Ultimately, Canon, Nikon & their fans are going to advocate in-lens stabilisation, where as Pentax et al are going to suggest in-camera stabilisation is far superior.
In practice it's better to try and avoid the movement in the first place than to manage movement once it's present.
One significant difference that no one else has explicitly talked about is the degree of stabilization possible with each system. As we all learned with the 1/focal length rule for minimum handheld shutter speed (Tv), lenses with longer focal lengths are more prone to blur because it takes much less movement to induce the same amount of blur than when using a wide angle lens.
Lens based Image Stabilization allows the system to be optimized for the focal length of the lens, and many telephoto lenses with IS can compensate for up to four stops with focal lengths in the 200-800mm range. This is the focal length range where it is needed most.
Camera based Image stabilization is dependent upon the speed and distance the sensor can be moved and is most effective with wider angle lenses. The servos needed to move the sensor fast and far enough to compensate for up to four stops with a lens in the 300mm+ range do not exist in a form factor that would fit in a typical DSLR body. Even if the sensor could move that far, the limit of the lens' image circle size would make that useless.