>>3159688Basically, the more zoomed in you are (that is, the bigger the focal length it is), the more shaky will the picture get. When everything is so zoomed, you can pan across the scene with very slight movements, so the tiny shake is therefore more obvious as well.
So - the advice is to use the shutter speed fast enough to eliminate the effect of the camera shake. That's what 1/focallength means - if you're shooting e.g. at 50mm, you should set your shutter speed to at least 1/50th of a second or faster. If it is 16mm, your shutter speed should be 1/16th or faster. Also, you need to keep in mind the crop factor, since that also comes into the play.
