This ihappensif the optical centers of the lenses are not exactly in the physical center plane of the lens as it is mounted inside the scope. What happens is that when the lenses that rotate turn about their physical axis, the optical center rotates off center, shifting the transmitted image in relation to the reticle.

To prevent this, one of two approaches is required-either the lenses have to be so perfect thattheir optical centers are exactly in the physical center of the lens mounts, or the power-change lens system has to be engineered so that it does not rotate as it moves back and forth inside the scope tube when you rotate the power change ring.

One quick way to determine if a given scope has such a problem or not is to insert a bore sighting collimator into therifle bore, then observe to see if the scope reticle appears to move when you turn thepower shifting ring. If the reticle does not appear to move, your scope is O.K. Of course, this test requires that the scope be mounted on a rifle......