What is Parallax?
Parallax, derived from the Greek word parallaxis (which means “alternation”), is the difference in the apparent position of an object viewed along two different lines of sight. In riflescope terms, it is the [apparent] movement of the scope’s reticle in relation to the target as the shooter moves his eye across the exit pupil of the riflescope. This is caused by the target and the reticle being located in different focal planes.
An Analogy on Parallax
Remember the dial-like speedometers in cars. Think about it being observed by the driver and the passenger of a vehicle at the same time. The driver, looking directly at the needle in relation to the numbers on the back plate, will get a more accurate reading than someone in the passenger’s seat viewing the needle at an angle. Think of the needle as the reticle and the back plate as the target. Adjusting the side focus to correspond with the distance to the target eliminates parallax at that distance.
One thing to note is that if you are looking through the optical center of the scope, parallax does not show up. It is only visible when your eye is off center. You may not be looking through the optical center of the scope if you’re shooting at sharp up or downhill angles. It will also affect you if your head is not positioned on the firearm’s stock properly.
The greater the distance to the target and the higher magnification of the optic means more parallax error.
While keeping the rifle still and looking through the riflescope, a slight nod of the head up and down will quickly determine if parallax is present. To remove it, start with the side focus mechanism on the number matching the distance you are shooting, then rotate in both directions, stopping when you see the target in focus. Note the reticle should already be in focus if you adjusted (link) the quick focus eyepiece properly. A common mistake is thinking the side knob will focus the reticle.
Move your head side to side and up and down to verify that the parallax has been eliminated. If it has, the reticle will remain stationary in relation to the target regardless of eye placement behind the optic.
Generally speaking, parallax should not be a major concern for the average big game hunter. If your scope happens to have an adjustment for parallax and you don’t have a need for it, you can set it at 100 and never touch it again. 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 or shoot drastically different yardages with the same scope.