Quote Originally Posted by DavidEccleston


The third grade of lenses have 3 way switch, with off, mode 1, and mode 2. Mode 1 is like the above. Mode 2 dampens motion vertically, but allows for horizontal motion for panning. Set the switch to the appropriate mode.


Just to clarify, on lenses in your 'third grade' with an IS mode switch, when it's set to mode 2 it dampens motion in the direction orthogonal to the panning direction, regardless of what direction you are panning. So, it damps vertical shake if you're panning horizontally to follow a car or bird, damps horizontal shake if you're panning vertically to follow a rocket or a rising elevator on the exterior of a building, and also damps vertical shake if you are holding the camera in portrait orientation for framing reasons and panning horizontally (so the camera is 'vertical').


Here's what Canon has to say about their IS systems.