http://www.longrangehunting.com/articles/what-is-parallax-1.php
Had this saved as a reference....may help.
There may be a modest amount of confusion out there on the subject of scope parallax. Parallax problems result from the image from the objective not being coincident with the crosshairs. (On high magnifications scopes, the objective is the big end of the scope; vice-versa for low power scopes; in either case it's the guzin end.) If the image is not coplanar with the crosshairs (that is the image is either in front of or behind the crosshairs), then putting your eye at different points behind the ocular causes the crosshairs to appear to be at different points on the target. (The ocular is the guzout end of the scope.) In fact, this is the basis of a test for parallax problems:
Set your scoped rifle on sand bags. Align the scope with the center of the target. Without touching the rifle, move your eye around behind the scope. Do the crosshairs appear to move on the target? If they do, the parallax is not set for the range of the target you are using.
So which way do we move the objective to correct parallax? First hold up the index finger of one hand in front of the palm of the other hand. (You don't have to actually DO it, this a thought experiment.) Let the index finger represent the crosshairs and the palm represent the image plane. If you move your head to the left, the finger moves to the right against the palm. So if your crosshairs move to the right on the target's image when you move your head to the left, the image plane must be further away than the
crosshairs. What's a mother to do? Why pull the image plane in a little by screwing the objective bell in so that the objective moves closer to you, of course. In this set up, the image is essentially tied to the objective so moving the objective 0.1 mm moves the image 0.1 mm. And no, the ocular
doesn't change this scenario any more than putting a weak loupe to your eye would change the sense of the thought experiment using index finger and palm. As long as we're on the subject of scopes, I might as well mention focusing the ocular or eyepiece (same thing). The goal here is to focus the ocular, which is really just a magnifying glass, on the _crosshairs_ which are located just ahead of the ocular. To avoid the distraction of the objective's image, you can cover the objective with something translucent like maybe a sheet of
Kleenex. Screw the ocular out, away from the main body of the scope until the crosshairs go out of focus. Now screw it in until the crosshairs are just in focus and then turn it in a little bit more. This puts the crosshairs slightly nearer than infinity as far as your eyes can tell. Your eyes will appreciate not having to strain to focus on the crosshairs, especially if they're old eyes like mine. Even if you have young eyes, a long day of varmint shooting will strain your eyes if you've focused your ocular by reversing the sense of the above procedure.
After you have focused your ocular, you can set your parallax by the procedure delineated in the above paragraphs. This is quite often a more accurate way of setting parallax than setting by the yardage lines inscribed on the objective bell (on many brands those lines are approximate at best).
