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Often, people will pick up and use technical terms without really understanding their meaning.
Take, for instance, the term parallax.
How often have you heard people talk about parallax?
Do you know what it means?
Can you explain it to other shooters?
If you can't, you're not alone. Many people are confused as to just what this term means.
Quite simply, parallax is the situation that occurs when the reticle and the image are not in the same optical plane, causing the image to be unclear or to move in relation to the reticle.
"Wait a minute, wait a minute," I can hear you say, "what's all this about optical planes?"
Remember when you were a kid and you would burn a piece of paper (or a bug) with a magnifying glass by focusing the light of the sun on it?
What you were doing was bringing the magnified image of the sun into the same optical plane as the piece of paper.
Therefore, when you are bringing the image of the target into the same optical plane as the reticle, you are doing something similar; you are focusing the image of the target onto the reticle. Bringing the image into the plane of the reticle is important because if it is not in the same plane, the image will not be clear and reliable. Just as you had to move the magnifying glass back and forth to focus the image of the sun on the paper or it would be unclear of blurry and not burn the paper, so you have to move the lens of the scope back and forth to bring the image into the reticle plane or it will be unclear and not be useful.
This is what we do in the Leupold factory when we set the parallax of the scope to a distance; we move the lens until an image that is a specific distance away, say 150 yards, focuses perfectly on the same plane as the reticle. You are doing the same thing when you adjust the adjustable objective or side focus dial of a scope with one of those features; moving the lens to focus the image on the same plane as the reticle. All of this is important because if the image and the reticle are not in the same plane, the image will be unclear and the reticle will appear to move in relation to the target. If either of these things happens, the result is obvious: you will not have a consistent point of impact. This becomes more important as the magnification of a scope increases - the greater magnification makes the focusing of the image more sensitive to changes in distance from the target. That is why lower magnification scopes are generally set at the factory for a specific distance and higher magnification scopes feature adjustable objectives or side focus dials. Knowing this and being able to explain it clearly will certainly inspire confidence in you on the part of other shooters. It will also help you in your own shooting activities, as you will better be able to understand how your own scope is performing.