How your eye affects a parallax-adjustable scope
by B.B. Pelletier
Lots of interest in this subject. I will do more on scopes, open sights and related topics in the future.
A repeatable cheek weld
Cyberskin, this is for you. We know that the relationship of the sighting eye to the sights is critical to either canceling or introducing parallax, but how do we ensure the same placement time after time? This is where the fit of the stock comes into play. When the U.S. Army used wood stocks with definite wrists, soldiers were taught to grip the stock with the hand their trigger finger is on in a certain way. That positioned the hand in the same place on the stock every time, and then the soldier was taught to place his cheek against the stock so it touched his hand. I'm right-handed, so I was taught to touch the upper heel of my palm (back of the thumb) with my cheek.
When we switched over to M16s, the wrist of the stock was straight and no one had figured out the hold yet. I bet it has since been determined, but I don't know what it is. Now, if you have a poorly fitted stock, as over 50 percent of them are these days (especially for using scopes!), there is an artificial way to ensure cheek placement. Figure out where you like to hold your cheek, then put a piece of tape on the stock at that spot. Every time you mount the rifle, put your cheek against that tape in the same way, and you will have solved the problem. Remember, cheek placement doesn't have to be perfect - in fact there is no perfect way to do it. It just has to be consistent.
On to the eyes!
All good scopes have an eyebell (ocular) adjustment to correct for individual vision problems. The instructions on how to adjust this focus ring are in the owner's pamphlet. If you're like me, you seldom read those things. Still, this is a critical step to getting the greatest precision from your scope - especially when it comes to parallax adjustment.
The eyebell adjustment is actually there to focus the scope on the reticle, so it appears sharp. Adjust it by looking through the scope at a blank, light-colored surface, such as a wall, and turn the adjustment ring until the reticle lines are in sharp focus. When you do this, look through the scope for only a few seconds; because, if you continue to stare, your brain will take over and focus the reticle for you. Just keep glancing though the scope for brief periods and turning the eyebell adjustment ring until the reticle is sharp.
How this works
You wouldn't think this adjustment has anything to do with parallax adjustment, and it doesn't - directly. But, indirectly, it has a tremendous affect! If your scope is not adjusted before you start shooting, every parallax adjustment you make will attempt to sharpen both the target AND the reticle. Since it is impossible to do both (they are in two different planes), you will tend to average the adjustment - to vary it so the target and the reticle both appear to be in the best relative focus. That will leave the target somewhat fuzzy, and there is no way you can guess at the amount of fuzziness on repeated tries.
Let me put this another way. If the reticle is out of focus, it will appear to be the most out of focus when the target is in sharp focus. In extreme cases, it may disappear altogether. So without thinking, you will back off on the parallax adjustment until the reticle appears somewhat clear again. And that is what throws off your ability to determine range using the parallax adjustment ring or knob.
Don't loan your gun!
The first thing a knowledgeable shooter does when borrowing a strange scope is adjust the eyebell to sharpen the reticle. When they give the gun back it's out of whack for the owner. It's easy to adjust it back, but how many shooters know to do it?
Let me tell you another horror story. I was displaying my field target rifle at an airgun show. A man picked it up and, without looking through the scope, began spinning the reticle knobs to feel the clicks. I hadn't bothered to zero the knobs or to note their settings and I had spent an hour optically centering the reticle. In a few seconds, all that work was undone. People who do things like that are called "knobdickers" and they exist in great numbers. So - don't loan your rifle, or be prepared for the consequences if you do!