The Image Stabilization

The image stabilization is the greatest innovation in lens technology after the introduction of autofocus. The IS allows to handhold every lens at a shutter speed 2-3 stops slower than what could be possible with a non-IS lens, and it greatly reduces the shake even when the lens is mounted on the tripod, in the case of super-telephoto.

 

Canon EOS 20D, Canon EF 600mm f/4 L IS USM, Canon 1.4x TC, 1/125 f/5.6, iso 800, handheld. Sile river, Italy. Even with image stabilization is it not easy to get a sharp photo handholding the 600 f/4 + 1.4x (840mm) at 1/125, but without image stabilization is would be completely impossible.
 
The stabilization was introduced in SLR photography in 1995, when Canon announced the 75-300 IS. The 75-300 was a cheap, low quality consumer lens - Canon often uses consumer tools as "test ground" for new technologies. This lens had the first generation of IS, that had an efficiency of 2 stops: in other words, it reduced the blur by the equivalent of a shutter speed two stops faster. The IS had to be turned off when the lens was mounted on tripod and during panning.  
 
A couple of year later, Canon announced the 300 f/4 L IS. It was the first L lens with image stabilization; even if it still had first generation IS, it had an additional switch to choose between IS Mode 1 and Mode 2. The Mode 2 is designed for panning, and it allows to follow the movement of the subject while using slow shutter speeds, while Mode 1 work well with static subjects.  
 
In 1999, Canon announced the new generations of super telephotos and the second generation of IS. The stabilizer used in the 300mm f/2.8 L IS USM, 400mm f/2.8 L IS USM, 500mm f/4 L IS USM and 600mm f/4 L IS USM has still an efficiency of 2 stops, but it can be used on tripod. All these lenses have Mode 1 and 2. The IS supertele are some of the finest lenses ever produced.  
 
In 2001, Canon announced the 70-200 f/2.8 L IS: the first lens with third generation IS. Other than the benefits of the previous version, the third generation is increased the efficiency to the equivalent of three stops. This IS has been added to many lenses, in two versions: the tele lenses (as the 70-200 or the 28-300) have 3rd generation IS with Mode 1 and 2, while short lenses (as the 24-105 IS) don't have the mode switch.  
 
Five years later, in 2006, Canon introduced the fourth generation IS with the 70-200 f/4 L IS USM. This version is identical to the third generation, but it increases efficiency to four stops.
 
 

How stabilization works

Before explaining how it works, it is necessary to clarify what it is. The "real" image stabilization moves an element of the lens, or the sensor, to counterbalance the camera shake. If there are not moving elements, it is not real stabilization: the "electronic" stabilization used in some video-cameras reduces image quality and it is works exclusively with video (it crops and moves each frame to create a smoother video); the "electronic" stabilization of some cameras is just fake. Many digicams have this fake stabilization (two examples are the "Picture Stabilization" of the Fuji F30 and the Casio "Anti Shake DSP"): it just raises ISO sensitivity, but it has nothing to do with real image stabilization.  
 
All the Canon IS lenses uses the real image stabilization; Nikon has an identical technology called VR, and Sony and Pentax uses a similar technology that moves the sensor instead of a lens element.  
 
The stabilizer is a ring-shaped device composed by two gyroscopic sensors, a CPU and a motor. The sensors detect the camera motion (due to hand shake, unstable supports, or other causes) on each axis (X and Y), then they send the data to the CPU of the lens. The CPU analyze and elaborate the data and it gives to the motor the instructions to move the lens or the sensor. The practical result it that the image projected on the sensor stays still, and you will get a sharp photo even if the camera was moving. Obviously, the stabilization has its limits. Nowadays, it allows a "sharpness gain" of 2-3 stops; in other words, the photo will be as sharp as if it was taken with a shutter speed 2-3 stops faster. For example, an image taken at 1/60s with stabilization will be as sharp as an image taken at 1/250s.

 

From left to right: entire photo, crop from photo taken with IS, crop from photo taken without IS.
 
There are many myths and truths about IS. The stabilization does not lower the image quality of the lens: stabilized lenses are as sharp as the previous non-stabilized versions, or even better. Of course, the stabilized elements adds complexity to the lens design, but the improvements in optical technology allows to maintain or improve the image quality of the previous versions. The stabilization is becoming more and more widespread, even in standard and short lenses (as the Canon 24-105 IS): it is very useful even into wide angles. If you are able to take a sharp shot at 1/20s with a 18mm lens, with stabilization you could use even 1/2s!  
 
Some brands (Sony, Pentax, Olympus) uses "sensor stabilization" instead of "lens stabilization". The principle is the same: the only difference is that the former incorporates the stabilizer into the lens, the latter into the camera body. The sensor stabilization has the obvious advantage that once you have bought a stabilized camera, you can use stabilization with all your lenses. The main cons is that you can not see the effect of stabilization into viewfinder, and some photographers argue that this kind of stabilization is a little less effective than lens stabilization. With the latter, you have to buy a stabilized lens, but you have the advantage of seeing the effect of stabilization into viewfinder.  
 
The stabilization reduces the blur caused by camera movements, but it can not reduce the blur caused by subject movements. If you are photographing a moving subject, and the shutter speed is very slow, you have to raise ISO sensitivity: the stabilization helps to reduce your movements, but only a fast shutter speed can freeze the motion of the subject. Other than that, remember that the stabilization needs nearly one second to activate: if you press immediately the shutter release, the IS won't give any benefit. You should always half-press the release for one second before the actual shot: when I photograph wildlife, I keep the release half-pressed for the entire duration of the action, to be sure of being always ready to shoot.
 
 

Stabilization and tripod

The stabilization does not replace the tripod. When you work with short lenses, as the 24-105 IS, you should shoot handheld only when you have a good reason to avoid the tripod (for example: with a very active and shy subject; when you are shooting from car, boat or plane; in places where the tripods are not allowed; when you really need to reduce at minimum the equipment weight; etc.). If you have the possibility to use the tripod, don't be sloppy and use it: the latest version of the stabilization corrects the camera shake by the equivalent of nearly 4 stops, but the tripod correct it infinitely ;-)  
 
The bigger lenses, as 400 2.8, 500 f4 and 600 f4 are often used on tripod, so you might think that stabilization is not necessary: that's wrong. These lenses are so big and heavy that they create small vibrations even on the sturdiest of the tripods, the long focal exacerbates every movements and, moreover, when you photograph action and wildlife the tripod head is never locked, so it is pretty easy to get shake. Nowadays, I'd never buy a super telephoto that has not IS.  
 
Using an IS lens on tripod raises some questions. First, does it affect image quality? With all the lenses announced after 1999 (from second generation), IS can be used on the tripod without problems. The only Canon lenses not compatible with tripod are the few first generation IS lenses still in production - the 300 f/4 IS, the 100-400 IS and the 28-135 IS. If you use another brand of lenses, and you are not sure about compatibility with tripod, I suggest to do this simple test: mount the camera on a sturdy tripod and take two exposures, one with stabilization and one without stabilization. If they are equally sharp, you don't have problems; otherwise you need to turn off stabilization when you use the lens on a sturdy tripod. In either case, the only risk is a loss of image quality - there is no risk to damage the IS unit if you forgot it turned on when you mount an first generation lens on the tripod.  
 
When you mount a "tripod-compatible" lens on the tripod, with IS turned on, you have to set the mode switch (if available) on Mode 1. Mode 1 is able to detect the tripod: if the giro sensors detect nil or very small vibrations, it "recognizes" the tripod and, after nearly one second, it switches to a special mode designed to correct mirror slap and shutter movement. Canon claims that "close examination of results from shooting tests definitively proves that there is no advantage to be gained by shutting off IS, or even by locking the mirror prior to exposure".  
 
This statement has proven true in my experience, but remember that you have to half-press the shutter release for at least one second before the exposure, otherwise you might get some blur because the IS doesn't have the time to detect the tripod.  
 
When the lens is mounted on the tripod, but the head is not locked and you move the lens to follow the action, the IS (Mode 1) works exactly as when you handhold the lens: it tries to correct relatively wide movements, both in horizontal and vertical direction. The same holds true when you use the lens on monopod or on other unlocked supports.
 
 

IS Mode 1 and Mode 2

Some Canon lenses have two IS modes. Mode 1 works both in vertical and horizontal direction, while Mode 2 works only in vertical direction, and it doesn't counterbalance the horizontal shake. The Mode 2 is necessary for panning: if you follow a subject with a slow shutter speed, IS mode 1 tries to counterbalance the motion, by moving the image in the opposite direction of your movement - the result may be loss of sharpness. Mode 2 instead correct only the vertical movements, so you are free to follow the subject.  
 
Usually, Mode 2 is the right choice for moving subjects, while Mode 1 works well for static subjects - when the camera is in landscape format, i.e. with horizontal framing. When the camera is in portrait format, i.e. vertical framing, Mode 1 is the only choice.