A More Technical Look
If you do not have the image of the target and the reticle in the same plane you will have parallax. You can tell if they are not in the same plane by moving your eye up, down or side to side. If parallax is present the reticle will appear to move off the target. A scope has several image planes and several focal planes and these vary depending on fixed vs. zoom and first focal plane scopes vs. second focal plane. For parallax you really only need to think of the scope as having two planes, one is where the image is formed and the second is where the reticle is focused. A target 1000 yards away will come to focus at a very specific distance behind the objective lens. A target at 100 yards will come to focus at a different location in the scopeís tube further from the objective than the 1000 yard image. The parallax adjustment on a scope simply moves the reticleís plane to be in the same location as the image plane. We are talking about very minute distances, like .1mm, which does not sound like much but it is compounded by magnification. Each power of the scope will multiply any error in parallax. So letís say you have adjusted the parallax the best you can and you missed aligning the image plane and reticle plane by .1mm, the .1mm misalignment would change as you change the power. For simplicity sake letís pretend we have a 1-20x scope (wouldnít that be nice!). We have it initially set on 1x and we adjust the parallax as close as we can but we missed aligning the image plane and reticle plane by .1mm, now we turn the magnification up to 20x and we have compounded the error x20. The misalignment now is a full 2mm off. And inside a scope dealing with planes, 2mm is a lot.
Generally speaking parallax should not be a major concern for the average deer hunter and if your scope happens to have an adjustment for parallax and you donít have a need for it, you can set it at 100 and mostly ignore it. Keep in mind that the yardage markings on the parallax adjustment are not exact and are just meant to get you in the ball park, fine tuning will need to be done to further eliminate parallax.
Parallax correction is a must for anyone attempting to use either a high magnification scope, anyone attempting to shoot drastically different yardages with the same scope or anyone attempting to shoot at extremely close ranges or extremely long ranges.
Common mistakes are to use the parallax adjustment in an attempt to focus the reticle. The ocular end (eye piece) has a focus for the reticle. That is the sole purpose of the ocular focus. Many returns are caused by users attempting to use the reticle focus as an image focus and or using the parallax adjustment as a reticle focus. If you get both of them so far out of whack bad things will happen. Only use the ocular focus to focus the reticle and only focus the reticle while looking through the scope at the sky. This will keep you from trying to focus an object instead of the reticle.
Here is a very good forum thread that has discussed the "how-to" of this issue in the past.