4 stops?

I know this sounds like a stupid question, but just so I understand it entirely, what does it mean when a lens with IS or OS is supposed to provide an extra "4 stops?"

When hand holding the general rule of thumb is that you set the shutter speed as the reciprocal of the focal length times the crop factor.

For instance, I'm shooting a 100mm lens on a 1.6 crop camera. The shutter speed should be around 1/160. A 4 stop IS allows you to reduce the shutter speed without introducing camera shake blur. A 4 stop IS would drop your shutter speed to 1/20.

As an example, this was taken at 300mm in a XTi with the 70-300IS at 1/20 shutter speed hand held.

Generally speaking the "4 stops" refers to the number of stops of shutter speed you can achieve hand-holding the lens. The "rule of thumb" is that the MINIMUM shutter speed required to get a shot without camera shake induced blur is 1/the reciprocal of the focal length. (Meaning, for example a 200mm lens should be hand held at a minimum shutter speed of 1/200). Four stops would mean that you could supposedly hand hold this lens at 1/15". (1/200' to 1/160' one stop, 1/160" to 125" two stops, 1/125" to 1/60" three stops, 1/60 to 1/15" four stops). This is in full shutter stops, my experience is rarely this. Technique has alot to do with this. Mine apparently sucks.

Theoretically speaking of course, YMMV.

Remember that this feature does no thing to stop the motion of a moving subject. Motion blur is a whole different animal.