I've noticed a lot of chat on the forums recently about IS — Image Stabilisation — so when the latest issue of EOS Magazine appeared in my letterbox with an article about IS, I took it as a sign.
Here's a short summary of some of the key facts in the article (from the Sep '03 issue).What Is it?
Image Stabilisation is a Canon solution to the problem of camera shake. It uses two gyroscopic sensors that sense the pitch and yaw (rockin' and rollin'
) of camera shake that adjusts the position of a movable set of lenses so the image on the sensor/film does not change.What happens...
...when you press the shutter button, if IS is on:
- The set of stabiliser lenses is unlocked from its central position.
- The gyros sensors start movement detection.
- Movement data is analysed and compensating instructions calculated.
- Compensation instructions are sent to the stabiliser lens group, which adjusts to counteract the camera shake.
- The sense-analyse-adjust sequence is repeated continuously until after the shutter button is fully depressed and the picture is taken.
IS Modes
There are currently 10 (13 August 2009: there are now many more including the EF-S fitting) lenses in the EF line-up that have IS. All the L lenses (eight of them) have two modes of operation: Mode 1 corrects for all shake. Mode 2 means the system does not attempt to correct for horizontal or vertical panning of the camera as the subject is tracked. The other lenses (EF 28-135 and EF 75-300) only have Mode 1 operation.
Chuck Westfall has made further posts including one that has this explanation about Mode 1 IS:
"Based on my own personal testing and observation, as well as a personal consultation with one of Canon Inc.'s chief executives in the Lens Division, these lenses (i.e., those for which IS can be left on when tripod-mounted) effectively have two IS modes within Mode 1. The decision on which mode will be used is made by the equipment, not the photographer, based on an evaluation of the degree of detected movement, as follows:
A. When the lens is mounted on a steady tripod and the shutter button is pressed halfway, the IS begins to operate immediately and the image in the viewfinder goes through a very slow vertical shift for about 1 second. After that, if the shutter button continues to be pressed halfway, the IS mechanism automatically goes into a special mode which is designed to detect and correct for mirror slap and shutter movement at slow shutter speeds. Based on my personal testing, it does this job spectacularly well. 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.
B. When the lens is mounted on a monopod or hand-held and the gyro sensors detect movement exceeding a certain rate, the IS mechanism switches into its conventional correction mode which is designed to compensate for relatively gross and/or erratic lens movement."
How much does it help?
IS gives two extra stops of shutter speed on all the lenses where it's incorporated, except the EF 70-200 L IS where 3 stops is gained. This means that, for example, instead of a shutter speed of 1/250 you could use 1/60 (2 stops) or 1/30 (3 stops). IS can handle shake from 0.5 to 20Hz. One Hz is one movement cycle per second. According to the article this means IS will counter mechanical vibrations, if you are travelling in a moving vehicle or helicopter.
Canon's Chuck Westfall claims that IS is effective until the shutter speed is twice the focal length (I guess for us 10D'ers that means twice the focal length times 1.6), and it doesn't matter whether you leave it on at faster speeds.
Tripods and Monopods and Other Stuff
The IS lenses are split between those where IS should be turned off for tripod use and those, which can work this out for themselves (the super-telephotos — 300/2.8L IS, 400/2.8L IS, 500/4L IS and 600/4L IS — the 70-200/2.8L IS and the 400/4 DO IS and the new lenses — the 28-300/3.5-5.6L IS, and 70-300/4.5-5.6 DO IS ). But in windy conditions and with the longer lenses, IS can still help. IS is great when you are using a monopod. Chuck also says that using IS on a tripod — when you shouldn't — won't actually damage the IS — only the picture. Do remember to turn off the IS before you remove the lens to avoid any possibility that the mechanism is still active.
IS lenses will work with the Canon 1.4x and 2x extenders. The article does list some other compatibility issues — nothing that is of interest to me and (probably) you, but if you really want to know, then you can take out a subscription.
13 August 2009, originally published 2004