I do not know if this answers the question asked, but it pertains to this area and is interesting, esp the last point, never heard of this one..... from the Leupold site...
An elliptical shape to the field of view, is brought about by having to use nearly all of the elevation and windage adjustment available in the scope. It is noticeable only at lower magnification levels and will not be discernable at a higher magnification levels. This does not affect the performance of the scope, nor will it in any way affect the point of impact. As the adjustments are brought back toward the center, the condition will disappear.
When an adjustment to windage or elevation is made to a scope to sight it in the adjustments actually move the erector system, which is a group of lenses housed in the erector tube. When an image passes through the objective lens the image is inverted or turned over. It is the job of the erector system to invert the image right side up. When an adjustment is made to the windage or elevation of a scope it is actually the image that is being moved not the reticle.
What is distortion?
Distortion is a general term referring to the situation in which an image is not a true-to-scale reproduction of an object, or a change in waveform shape takes place. There are many types of distortion.
Centering of a scope's adjustment dials
The elevation and windage adjustments of a scope are easily centered. Place a small mirror against the objective end of the scope. That would be the end farthest from your eye as you look through the scope. Make certain that the mirror is large enough to cover the entire objective. It must also be flat against the objective. With the scope's power selector ring set at the lowest magnification, look through the eyepiece as you would while aiming at a target. If the scope's windage and elevation adjustments are off center, you will see two images of the reticle,cross-hair). To reach the center of the adjustment range, simply turn the elevation and windage dials until you see only one image of the reticle.
AND on another site....
What is MOA, and what amount should I get in my scoperail?
MOA stands for Minute of Angle, and is a measurement of an arc of a circle. A circle in generally broken down into 360 degrees. Each degree can be further broken down into 60 minutes of a degree of angle. And further than that each minute of a degree can be broken down into 60 arc seconds of a minute of angle. So, 60 arc seconds = one minute of angle, 60 minutes of angle = one degree, and 360 degrees completes a circle.
MOA is applied to your scoperail by making the front of the scoperail lower than the rear of the scoperail. When your scope is mounted to this "sloped" rail, it is mounted at an angle relative to your riflebore centerline. Since the scope is what is "pointed" at the target, the barrel is "pointed" at a higher verticle position than your target. You might be asking "isn't the internal scope adjustment supposed to do this"? The answer is yes, to some degree,excuse the pun). Your projectiles travel in the air to your target in an arc we call trajectory. The further the taget is, the higher the arc needs to be to reach the target. From a 100yd zero we need to add 25 to 30 MOA to our scope's elevation adjustment to hit a target at 1000yd. Shooting farther requires adding more elevation to our scope's adjustment. Since the average scope has about 80 MOA of elevation adjustment,which equals about 40 MOA from center adjustment to max elevation) we will run out of elevation adjustment at 1200 to 1500yd. Additional to that, at the extended ranges where we need a very clear sight picture, the erector tube,part of the scopes internal adjuster) will have us looking through the edges of the objective lens. Our best sight picture is when our elevation and windage adjustments are nearest to center. By angling the scoperail we not only add to the overall elevtion adjustment possable, we are looking through more of the center of the scope which will give us the best sight picture.
In general, if you plan on shooting out to 800yd and no more, a flat scoperail will be best. If you plan on shooting 200 to 1000yd a 30MOA scoperail will be best. If your shooting will be from 600 to 2000+yd a 60MOA scoperail will be best. If you plan on shooting past 2000yd, we may need to build a custom scoperail for you. It is important to review your scope choices before choosing a scoperail since not all scopes have the same amount of adjustment or light gathering abilities,which is another topic in itself).
