What is an Optical Image Stabiliser?

The Basics:

An optical image stabiliser is a system which compensates for camera shake, so as to reduce the incidence of blurry photos in handheld photography.  In Canon cameras, this is achieved by analysing movement with gyro sensors, then shifting a lens element within the lens to correct the light path. The benefit of this is that the photographer can shoot at speeds up to 4 ‘stop’s slower (depending on product), and still get a sharp shot. You can tell that a Canon lens has a stabilizer by the initials ‘IS’ in the name.
See Camera Shake for more information.