Dave,
Hope this helps:
(from
http://www.usoptics.com/sub_pages/reticles.php )
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Only in a variable power scope is the reticle placement a major problem. In the rear focal plane, or behind the power changing lens system (erector tube), was the first solution that occurred to optical engineers, and most American scopes are still being built that way. Unfortunately, this apparently ideal solution has a very serious flaw.
Any tolerance change in the centration of the lens system and their spherical/longitudinal movement with the power change, will shift the point of impact. A variation of one thousandth of an inch will move the zero point approximately one inch at 100 yards. Since the mechanical parts that hold the power changing lens system slide inside each other, (some allowances are made for temperature changes, manufacturing tolerances and wear), there must be some movement made to accommodate this. Consequently this lateral and vertical movement will often shift zero by as much as several inches as power is changed.
A better solution is to place the reticle in the front focal plane, or ahead of the power changing lens system. The movement of the erector system will, optically, have no effect on the point of aim here. So why don't all scope manufacturers build them this way? The downside of this method is that Americans typically do not like reticles that grow in size when the power is turned up. There is no actual growth in the reticle size. As the magnification increases, so does the reticle along with the objects in the field of view. A one inch dot reticle will still be one inch, at any power, be it low or high. It is only the appearance that is altered. If the power is turned from 2x to 4x, or doubled, the size of the objective image is doubled, and so is the reticle along with it.
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