Now we all know (or I hope we all know) that the general rule of minimum shutter speed to avoid blurring a shot with camera shake is generally held to be 1 over the lens focal length - ie 200mm lens it has to be a minimum of 1/200th of a second.
Now a friend of mine swears black and blue that for a zoom lens, let's say 50mm - 200mm for the sake of the argument, it is the "maximum" focal length you use - ie 200mm or 1/200th of a second in this case.
My argument is that if the lens is set to say 50mm it is 1/50th of a second.
forgetting overly technical "what ifs" such as a heavy zoom lens is harder to hold still than a prime lens, but some of the cheaper zooms are not overly weighty and some professional limited zoom lenses (say 16mm - 35mm) can weigh as much as cheaper long zooms. My argument is that it comes down to the image size on the sensor which will be the same on a 50mm prime lens as for a zoom set to 50mm. Put another way the image of something with a 200mm lens will be as equally blurred by shake as the same image at 50mm - but because the image is so much smaller it will be appear sharper.