Until relatively recently photographers hand-holding telephoto lenses faced a dilemma. As lenses increased in power and weight they became increasingly difficult to hold steady enough to avoid image blur. The quandary was solved with the application of some ingenious technology – Image Stabilization. CPN reveals the story of how this ingenious Canon technology was developed.
Any photographer who has used a camera fitted with a powerful telephoto lens will understand the term “image shake”. The more powerful the lens, the more any small vibrations are magnified. The excitement at the prospect of filling the frame with distant objects quickly wears off as the frustration of an uncontrollably jumpy subject and the reality of blurred images sets in.
Hands typically shake at between one and five tiny movements per second, and the lens in a photographer’s grasp shakes with them. This has the effect of moving the subject off the lens’ optical axis, resulting in the appearance of a jumpy subject, and movement of the subject across the film or image plane during exposure time. The magnification of the lens works to exaggerate this effect.
Unless working in very bright conditions most people will find lenses with a focal length of more than 300mm difficult to use successfully without the use of a tripod. Tripods, however, can be cumbersome, heavy and bulky – hardly an ideal or practical solution in every situation.
Tilt System IS
Canon addressed the problem with its Image Stabilisation technology that was first seen in the EF75-300mm f/4-5.6 IS USM zoom lens in 1995. A special corrective lens group was developed to sit behind the objective lens group. This IS lens group is designed to refract or ‘bend’ light rays when it is moved perpendicular to the optical axis.
When a lens is moved off its optical axis by vibrations, a detection system consisting of two tiny gyros leaps into action. Highly responsive coils move the IS lens group by precisely the right distance in the direction of the vibration. Because vibration can occur in both horizontal and vertical directions, design of the corrective optical system allows movement up and down and left to right.
Hair trigger response
All of this starts to happen within approximately 0.002 of a second of a vibration being detected. This essentially immediate response allows the IS system to move the correction lens group with the vibration, effectively cancelling image shake effect.
IS: the faster lens
Image Stabilisation is so effective that it allows reduction of shutter speed by between two and five stops without any perceptible increase in image blur. Initially it was a two stop reduction but this was changed to three with the arrival of the EF70-200mm f/2.8L IS USM in 2001; to four steps with the EF70-200mm f/4L IS USM in 2006; and finally five steps with the EF200mm f/2L IS USM in 2008. This has obvious implications for low light photography, or photography where using a tripod is impractical or undesirable.
Using IS with a tripod
When using certain early models of IS lenses with a tripod it was necessary to turn off the IS function. This is because of a phenomenon known as ‘Shake Return’. Shake Return occurs when the IS system tries to correct vibrations to which the system itself contributes. When the IS lens sits on a tripod, the IS detection gyros pick up any tiny vibrations or movement; these might be caused by the tripod being knocked, or the photographer adjusting a camera setting.
The IS system then swings into action to correct that movement. The movement of the IS lens group causes its own minute vibration, which is in turn detected by the movement sensor, which triggers another correction. This ‘feedback loop’ can continue endlessly, resulting in the addition of unwanted blur to images that would be sharper if the IS function was switched off.
Canon addressed the ‘feedback loop’ in later model IS lenses by introducing an algorithm to the IS detection system to automatically recognise when the lens is mounted to a tripod. When these lenses are mounted on a tripod and the shutter button is pressed halfway, the IS system kicks in and the image in the viewfinder can be seen to go through a very slow vertical shift for about one second.
If the shutter button remains depressed halfway the IS system detects the lack of motion and automatically switches into a special mode. In this mode IS detects and corrects for mirror slap and shutter movement at slow speeds, but not for ‘normal’ lens shake. There is no advantage to be gained in turning off the IS function or locking the mirror prior to exposure.
The early model lenses which do not have this automatic function and which should have IS turned off (ie lock the IS correction lens group in place) when mounted on a tripod include the following lenses:
- EF28-135mm f/3.5-5.6 IS USM
- EF75-300mm f/4-5.6 IS USM
- EF100-400mm f/4.5-5.6L IS USM
- EF300mm f/4L IS USM
Using a monopod
When an IS lens is mounted on a monopod or hand held the IS mechanism detects the ‘slight’ yet inevitable movements as relatively large and erratic and therefore switches into its conventional correction mode, compensating for any detected movement.
The trouble with panning
The stabilisation characteristics of the IS system are most effective when shooting stationary subjects. When panning a moving subject is attempted the IS system attempts to correct the movement, the act of which can detract from rather than enhance a photograph. To resolve this problem Canon developed Image Stabilizer Mode 2.
When shooting in this mode, if a major movement in a particular direction continues for a certain time, the Image Stabilizer system determines that this movement is conscious panning, rather than camera shake. Correction in the relevant direction halts, while correction perpendicular to the panning direction continues. In other words, the system detects panning operation and executes the appropriate control suitable for panning, resulting in a sharper image of the subject being panned. First seen on the EF300mm f/4L IS USM Mode 2 is now seen on other Canon lenses, mainly telephotos and telezooms.
Canon’s IS lens history
|1995||EF75-300mm f/-5.6 IS USM|
|1997||EF300mm f/4L IS USM|
|1998||EF28-135mm f/3.5-5.6 IS USM, EF100-400mm f/.5-5.6L IS USM|
|1999||EF300mm f/2.8L IS USM, EF400mm f/2.8L IS USM, EF500mm f/4L IS USM, EF600mm f/4L IS USM|
|2001||EF70-200mm f/2.8L IS USM, EF400mm f/4 DO IS USM|
|2004||EF28-300mm f/3.5-5.6L IS USM, EF70-300mm f/4.5-5.6 DO IS USM, EF-S17-85mm f/4-5.6 IS USM|
|2005||EF24-105mm f/4L IS USM|
|2006||EF-S17-55mm f/2.8 IS USM, EF70-200mm f/4L IS USM|
|2007||EF-S18-55mm f/3.5-5.6 IS, EF-S55-250mm f/-5.6 IS|
|2008||EF-S18-200mm f/3.5-5.6 IS, EF200mm f/2L IS USM, EF800mm f/5.6L IS USM|