Camera shake is the thief of sharpness. If you are hand-holding a camera and lens, they will move as you press the shutter release. Movement during exposure blurs the image.
Much of the time, you will not notice the effects of camera shake. If you are shooting with a fast shutter speed or a wide-angle lens, the blurring may not be significant− but it will still be there, and might appear if you have a big enlargement made from the image.
The only way to overcome camera shake is to eliminate the movement of the camera and lens during the exposure. The obvious way to do this is by taking the camera out of your hands and fixing it to something that will not move, such as a photographic tripod. However a tripod is only effective if it is sturdy, which usually means heavy.
Fortunately, Canon offers another method of reducing, if not eliminating, the effects of camera shake. Image stabilised lenses, first seen in 1995, approach the problem laterally. Rather than trying to stop the movement of a hand-held camera, they seek to introduce an opposing movement within the lens. The aim is to keep the image static on the sensor or film, despite the movement of the camera.
Image stabilisation was available for camcorders long before it was introduced in EF lenses. Both electronic and optical systems were available, but size or weight constraints meant that neither was suitable for EOS cameras. So Canon went back to the drawing board and took a fresh look at the problem.
Its solution is a group of elements inside the lens that moves perpendicular to the lens axis. The movement of this special lens group is controlled by an on-board microcomputer and counteracts the shaking of the camera.
The following sequence of events takes place when the camera shutter release button is partially depressed:
When the first EF lens with IS was introduced, it was the first time that a high speed 16-bit microcomputer had been incorporated in a lens. The computer simultaneously controls the Image Stabilizer, Ultrasonic Motor (for focusing the lens) and the electromagnetic diaphragm (for setting the lens aperture).
Image stabilisation is effective with movement from 0.5Hz to 20Hz (1Hz is one movement cycle per second). This will cope not only with situations from simple camera shake (0.5Hz to 3Hz), but also the engine vibrations encountered when shooting from a moving vehicle or helicopter (10Hz to 20Hz).
There is no reduction in the optical performance of the lens.
Power for image stabilisation lenses comes from the camera battery, so there will be fewer exposures per battery charge when an image stabilisation lens is attached to the camera and switched on.
Camera shake is detected by two gyro sensors in the lens; one for yaw and one for pitch. The sensors detect both the angle and speed of the movement.
When the camera is static, the rays of light pass through the lens and form an image on the film or digital sensor.
When the camera moves, the rays of light from the subject are bent relative to the optical axis and the image shifts slightly on the film or digital sensor. You can see this effect in the camera viewfinder if you gently shake the camera while viewing a subject.
With an IS lens, the gyro sensors detect the camera movement and pass the data to a microcomputer in the lens. This instructs a special group of lens elements to move at right angles to the lens axis. The amount and direction of this movement is just enough to counteract the amount and direction of the camera shake. The result is that the paths of rays of light passing through the lens are adjusted so that the image remains stationary, relative to the film or digital sensor.
One problem with the first two EF lenses with IS produced is that the system regards panning as camera shake, and tries to overcome it. This causes the viewfinder image to jump about, making it difficult to see and frame the subject accurately.
On the later lenses, you have the option of two IS modes. Mode 1 is the same as before and is used when you are shooting static subjects.
Mode 2 can be set when following a moving subject with the camera (panning). The lens detects the sweeping movement and switches off the IS correction in that direction (horizontal or vertical). IS correction in the direction perpendicular to the panning movement continues as normal to help give a sharper image.
IS Mode 3 was announced with the EF300mm f/2.8L IS II USM and EF400mm f/2.8L IS II USM lenses, and is also in the EF400mm f/4 DO IS II USM, EF500mm f/4L IS II USM and EF600mm f/4L IS II USM telephoto lenses. The EF100-400mm f/4.5-5.6L IS II USM and EF200-400mm f/4L IS USM EXTENDER 1.4x telephoto zoom lenses also feature Mode 3. IS Mode 3 takes the benefits of standard IS (effective for both horizontal and vertical camera motion) but, instead of it being active all the time, it only activates when you fully press the shutter button to take an image. It is especially useful for sports photography where you are likely to be moving between subjects quickly.
In IS Mode 1 this can create a bump or jump within the viewfinder as the IS motor races to keep up with extensive lens movements. Instead, by not activating until the shutter button is fully pressed it saves the system trying to compensate for random, rapid lens motion and only compensates at the point you are taking an image.
Introduced with the EF100mm f/2.8L Macro IS USM lens, Hybrid IS takes the image stabilisation concept and applies to it to macro photography. When using longer lenses or for general purpose shooting, any camera shake appears to be rotational - i.e. an up and down or side-to-side movement around a point - that point being the camera. This is effectively corrected by the IS motors contained in the lenses. However, when you move in close for macro photography the camera shake motion appears to be less rotational and more shift based - as if the whole frame is shifting up and down or side-to-side parallel to the subject. This is what Shift IS, found in the Hybrid IS system of the EF100mm f/2.8L Macro IS USM lens, aims to correct.
The earliest IS lenses give a gain of about two shutter speed steps. This means, for example, if you shoot with a shutter speed of 1/60th of a second with IS, it will be the equivalent of 1/250th of a second if you shoot without the Image Stabilizer switched on. Or if you can obtain a sharp image without image stabilisation at a shutter speed of 1/60th of a second, you will produce results of similar sharpness at 1/15th of a second with image stabilisation, other factors staying the same.
One of these factors is focal length. Increasing focal length not only magnifies the subject − it also magnifies the effects of camera shake. A useful guide is that you should use a shutter speed at least equal to the reciprocal of the focal length when holding the camera and lens by hand. So if the focal length of the lens is 200mm, the shutter speed should be at least 1/200th of a second.
More recent IS lenses have improved their effectiveness, giving a three-step, four-step or a five-step gain (a five-step gain is found in the EF200mm f/2L IS USM lens). A four-step gain means that shooting with a shutter speed of 1/15th of a second with image stabilisation gives the same image sharpness as shooting at 1/250th of a second without image stabilisation.
Recent lenses also have the IS system up and running within 0.5 second of partial pressure on the shutter button, rather than the 1 second delay with earlier lenses.
Keep in mind that image stabilisation only reduces the effect of camera shake − it has no effect on subject movement.
IS lenses work with all EOS cameras, regardless of when they were introduced. The IS system − gyros, microcomputer and special lens group − is part of the lens, not the camera. However, there is some communication with the camera and there are some operational differences between the EOS SLR models.
We do not have full data for every IS lens/EOS camera combination. Before taking a picture, look through the viewfinder to check that the IS system is operating correctly (remember to allow up to 1 second for the system to start up after the shutter button is partially depressed). If there does not appear to be any increase in the steadiness of the image, or the image is jumping around in the viewfinder, there is probably an incompatibility. Movement of the image in the viewfinder immediately after an exposure is not usually a problem and will not affect the sharpness of the exposed image.
IS lenses work well when used with accessories. Add an EF extender and you can increase the effective focal length of the lens by 1.4x or 2x. Increased focal length means that the effects of camera shake are also magnified, so the image stabilisation is very useful. EF extenders are compatible with L-series and DO lenses of focal length 135mm and greater, and a small number of wide-aperture telephoto zoom lenses.
Image stabilisation also remains effective when extension tubes or close-up lenses are used.
Although an IS lens gives more opportunities for hand-held shots, there will still be times when the support of a tripod is needed − with exposure times of several seconds, for example, or when working with heavy EF500mm or EF600mm lenses.
With some of the earlier lenses, you need to switch the IS off when using a tripod. The lack of movement confuses the system and the image starts to jump around the viewfinder. However, using a tripod in high wind or with super telephoto lenses, often results in some camera movement, and the IS system can be invaluable. Later IS models are able to sense the use of a tripod and automatically disable the IS, if necessary.
You should also leave the Image Stabilizer on when using a monopod, as it is unlikely you will be able to keep this type of support perfectly still.