Friday, April 2, 2010

Understanding and Correcting Parallax

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You see the term often used when describing scopes, but what exactly is parallax, and do you need to concern yourself with the parallax settings of a scope? Parallax describes a situation where the focal plane of the object in the scope is offset from the reticle. If you have parallax, you have an optical illusion that must be corrected. Parallax should not be confused with focus. Parallax compensation changes neither the focus of the reticle nor the focus of the image, but simply moves the planes at which these two objects are in focus so that they are coincident (sharing the same plane).

When looking through a high power scope you can identify parallax by adjusting your gaze slightly. If the reticle changes it's position on the target when you shift your gaze, your parallax is not properly compensated for at that range. The reticle appears to move in relation to the target image because the focal planes are not coincident, making it appear to be a 3D image. Properly adjusted, the reticle should appear to be locked in place as if it were painted onto the target so that no matter how your gaze is shifted the reticle position never changes relative to the target.

Consider the image below:

In this illustration, the point where the focal lines cross and form an X is the location of the focal plane for the target image. Note that it is behind the reticle. When the angle at which you are viewing the image through the eyepiece changes, the reticle position relative to the target image will also change.

In this illustration, the focal plane for the target image and the reticle are the same, so no parallax adjustment is necessary. It is possible to have an accurately placed reticle in the first image only if you are looking at through the scope with your line of sight exactly lined up with the reticle and target image. The problem occurs when your line of sight is not exactly lined up, as the point of aim indicated by the reticle is now incorrect. By eliminating parallax and having the target image and reticle on the same plane, you no longer have to have a precise line of sight: no matter what angle you are looking through the scope at the reticle will still accurately indicate the correct point of aim.

Parallax is usually negligible or not present at all in most low-magnification tactical style scopes, as the scope is too short or the range is not long enough. 1x red-dot style scopes generally are parallax free at any range. Even mid-power hunting scopes have very little parallax, and many tactical models such as the UTG 1.5-6x40 illuminated reticle scope do not have parallax compensation, as it is impossible to quickly and accurately determine range in a dynamic tactical situation.

In high-power scopes used over long distances it is necessary to compensate for parallax. High power scopes are usually equipped with a side-mount turret, or adjustment ring located on the objective bell, so that the shooter can move the focal plane of the target and reticle and eliminate parallax. Some of these rings or turrets are marked with various distances, generally ranging from 50 yards to infinity, to indicate the proper setting to eliminate parallax. While helpful as a general starting place, these factory set markings are not always accurate. I find it helpful to manually determine the proper setting at 50 yard increments and scribe or mark those settings on the scope.

Why is it so critical to get a precise parallax compensation setting? The reason is that the amount of parallax increases with magnification, giving you a larger margin of error at higher powers if your parallax is not precisely corrected. On a high-power variable 6-20x magnification scope for example, parallax appears easy to compensate for at the lower 6x magnification setting. Once the zoom is increased to 20x however, it takes a very fine adjustment to completely eliminate parallax.

To measure the actual parallax compensation needed for a given distance and zoom, you will need to head out to a range with known distances and calibrate your parallax adjustment mechanism. Some parallax adjustment systems such as a side-mounted turret have knobs that can be adjusted to "rezero" the mechanism. In order to get consistent parallax compensation, start with the adjustment knob or ring set on the stop past "infinity".

Starting with a scope that has already been zeroed to your rifle, set up the firearm in a stable configuration aimed downrange at your target using sandbags or a machine rest. Your target should be at the maximum distance possible at your shooting range- preferably 1,000 yards, though shorter distances will suffice. With the magnification set to maximum, sight through your scope and slowly shift your gaze while looking for movement of the reticle in relation to the target. Use the parallax compensation turret or objective ring to adjust the focal planes until there is no movement of the reticle when you shift your gaze. Then, using a fine paint marker mark the point on your ring or turret for this range.

You can continue this process at various distances by moving the target closer and repeating the process. If you wish to do so, you can permanently scribe the positions on your ring or turret so that it is easy to consistently return to that point time and again. Remember, when adjusting parallax using a side-mount turret, always start with the turret set against the stop past infinity and then turn it to the appropriate setting. This ensures that there is no slack or backlash that can throw off your adjustment.

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