The gererally accepted focal length for what you see with you eyes is what a 50mm lens "sees." Therefore 100mm =2X, 150=3X etc, based on what your eyes see. However, the generally accepted lens formula for optical zoom is the lowest lens focal length divided into the highest focal length.
When you consider an ultrazoom camera (more than 6X optical zoom) IS or image stabilization should be used with it because when zoomed out to the maximum in the case of a 10X optical zoom equipped camera, The slighest in voluntary movement you make while holding the camera is magnified 10 times in your photo.
Therefore, IS should be a top priority consideration.