AR15Barrels, that issue of focus and parallax mismatched can happen. The cause is the range scale on the adjustment's not exact for the actual objective lens' focal length.
Let's say the scope's optical system designer chose the objective (front) lens system to have a 200mm focal length. There's a tolerance in making lenses; ones of that type can easily have a few millimeters spread. The mechanics of the focus system are exact; a given threads per inch of the adjustment moves the lens mount exactly the same for all scopes with that design. So the range marks can be put on each and they'll all be perfect for the designed lens focal length.
But the one in your scope is different. You set your range focus at 200 yards but the objective lens actually focuses that target in front of the reticule if it's actally 198mm, behind the reticule if that's a 202mm lens.
Depending on how much error the objective lens' focal length is off from spec, the amount of parallax will vary.
Note that if you're teaching people to not focus their scopes, they'll never be able to adjust parallax. These terms both relate to the same thing. It's physically impossible for the reticule be properly focused on by the eyepiece and when the range focus is on the reticule, to have any parallax at the target range. If I understand what you're saying, I think the scope's eyepiece wasn't focused perfectly on the reticule but instead in front or behind it.
What did you use to "focus" the scope on the target? I ask because all the scopes I've seen only have one adjustment to focus at the target range and it cannot be the eyepiece.
What make and model of scope do you have? This interests me very much and I'd like to look into it deeper.....thanks.
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Former US Navy & Palma Rifle Team Member
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Last edited by Bart B.; February 26, 2013 at 11:29 AM.