From the US Optics website:
Reticles: Front or Rear Decisions
Introduction and History There are two planes of focus in the common rifle scope (lensmatic), for the placement of the reticle. They are commonly called the front focal plane and rear focal plane models. One exception is the Shepherd scope, which has both. Artillery rangefinders have always had at least two reticular focal planes, and sometimes three or four. Optical collimating scopes have always had two focal plane reticles, or aiming points. Interferometers need more than one focal plane aiming point to function.
Why front or rear focal plane placement? Question: What are focal planes and what is the difference between putting the reticle in the front or rear focal plane? Answer: 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.
Since the front focal plane reticle is a superior aiming device but aesthetically not very popular, there is only that problem to overcome.
That problem has been solved by U.S. Optics engineers in the form of creating a series of front focal plane reticles that do not appear to change in apparent size as the power is changed. These reticles all have the same effect when sighting with them. U.S. Optics designs these reticles to not only diminish the negative idea of apparent change, but uses that concept to create an exclusively positive concept change. In other words, we use the single image concept of a reticle magnified to an almost unusable thick, heavy image at high power to create another entirely different and very usable, highly magnified reticle, without the normal disadvantages. We call this system of reticles our High-Low Imaging System, or High-Low Reticle. It is a completely different picture at high power, thus usage is dual purpose and increases the versatility of the scope tremendously.
With this system, the variable power scope no longer has any disadvantages, and many decidedly great advantages over a fixed power scope.
