One question I receive often is " does my point of impact, or zero change as I vary magnification on my variable powered scope"? Most often the question comes from a shooter who is seeing a POI change when magnification is changed considerably.
On a mechanically sound scope, point of impact should not change as magnification is increased or decreased.
What can cause POI to change with magnification?
If a rifle is zeroed at say 50 yards, then the target is moved out to 100 or even 200 yards, and the scope is zoomed for the increased distance, the shooter may not be able to focus as finely on the desired point of impact as well as he could at 50 yards (bullet drop not included). This could cause the point of aim to change explaining the change in point of impact.
Even at less distance change with only a slight change in magnification this can occur. This can even occur with no change in magnification.
The same thing can occur if zeroed at long distance then the target is moved closer with less magnification.
This sounds simple but can explain why a shooter may think his scope is changing point of impact with changes in magnification, but really isn't.
Mechanical issue with the scope
Any scope with a good design and that is mechanically sound should not have a shift in point of impact when magnification is changed. Some very cheap scopes can exhibit this issue simply because the design is not sound. If a top of the line scope is displaying this problem it is probably because it has been damaged in some way. Dropping it, etc.
The issue is generally seen more on scopes that have the reticle in the second focal plane (SFP) versus the first focal plane (FFP) but this doesn't mean that all SFP scopes will have the problem. It's just that the design is more susceptible to having this problem.
What is SFP vs FFP?
FFP stands for first focal plane. This means the reticle you see (the cross hairs and other marks) are printed or engraved on the first focal plane lense. If you can look through your scope and change magnification and the reticle markings do not change in relation to the target, your reticle is in the first focal plane. See the diagram below.
A second focal plane scope is one where the reticle is on the second focal plane. If you can zoom magnification and your reticle stays the same and doesn't change like in the diagram above your scope is an SFP scope.
When purchasing a scope, always check to see which focal plane the reticle is in before you buy. I own both FFP and SFP scopes. My general rule is, if I am going to be using the rifle and scope combo at distance that require large hold overs, I always go with an FFP scope. If I am only going to be shooting within the capable distance of the cartridge, I go with a good SFP.
Why aren't all reticles in the first focal plane?
FFP scopes allow the shooter to use the marking or dot in the scope for hold over no matter what the magnification. SFP scopes do not. Markings or mildots will only be accurate on one certain magnification. That magnification is given by the manufacturer usually in the documentation that comes with the scope.
Typically first focal plane scopes are more expensive than second focal plane. In an effort to remain price competitive and give all shooters what the seek, most manufacturers offer both.
A second focal plan scope is more likely to give you a change in point of impact when magnification is changed but technology has improved substantially over the past few years. This problem is usually associated with older scopes, not because of their age but because of the technology at the time of manufacture.
How do I test my scope to see if this problem exists ?
Whether your scope is FFP or SFP, you can test it to determine if point of impact changes with magnification.
Mount your rifle in a vice or lead sled so that it doesn't move while you work. in front of the rifle mount a blank sheet of paper on the wall or some stable platform to hold it still. To get an accurate reading it is important that nothing moves during this procedure.
Have someone use a good flashlight and shine it into the ocular end of the scope. It will take a little moving it around before you find the right spot. When the flashlight is in the correct position you will see the reticle on the sheet of paper.
Adjust the parallax and magnification until the reticle on the paper is as sharp as possible. You may have to increase or decrease the distance between the scope and the paper at this point in order to get a clear view of the reticle and / or you can try adjusting parallax and magnification. Just get a clear picture of the reticle on the paper.
Make a tiny mark on the paper where the crosshairs intersect. Large enough for you to see when standing next to the rifle. Then change the magnification on your scope slowly. It's usually best to start at low magnification then work up to high.
If the intersection of the crosshairs move off of the mark you made there is a mechanical problem with your scope, or your scope's design is just not good enough for superior accuracy.
Make sure nothing moves while you are increasing magnification. If you see movement of the crosshairs it probably won't be much. You will have to decide for yourself if it is too much for your goals. A little movement can move POI a lot at long distances.
To conclude, your scopes primary cross hairs, the ones you used to zero, should be right on target despite changes to magnification. This should be the case no matter which focal plane your reticle markings are on. If you are sure this is not the case, and the amount of movement is more than you can tolerate and your are sure poi is not changing because you cant see the target as well on lower magnification, get rid of the scope you have, or have it repaired. It is not functioning properly.