The term parallax is used a lot when we are dealing with rifle scopes and optics. With some optics like red dots using a lack of parallax as a selling point. But what is parallax and why should you be concerned about it?
What Parallax Is
Parallax is a phenomenon dealing with how you perceive objects in space. This occurs when you have objects on different focal planes. For example, let’s imagine there is a pole a few feet in front of you. This is the second focal plane you see with your eye being the first focal plane.
Then directly in line with it, another pole is placed which will be your third focal plane. If you look at the first pole just right, you can cover up the farther pole with the first pole. If you hold this position the two poles will always be correctly in line with each other.
However if you shift your line of sight, the poles’ orientation changes. It might look like the poles are misaligned or are moving from being aligned with each other. The poles haven’t moved but the orientation of your eye with the poles has changed.
This shift or misalignment is called parallax. You have to deal with it in some way whenever you look through a pane of glass. Sometimes the parallax is more noticeable, other times it doesn’t present itself as easily. This is usually more noticeable when a level of magnification is present.
Why is it important?
Parallax can result in some unique problems, especially when dealing with longer ranges. If you don’t consistently place your head behind your optic you are more likely to get a parallax effect between the reticle and the target.
This applies mostly for rifle scopes and less for quality red dots. If parallax is present, you might try and align the reticle with your target. The reticle might already have been perfectly in line with the target, but since it didn’t look like it was you moved the reticle over, throwing off the shot.
Shifting the reticle can result in a miss at best or worse cause you to hit something unintentionally. For the most part, this would be an extreme example of parallax. If you can consistently place your head behind your optic, parallax will be a minimal issue. But higher end scopes do have a parallax adjustment just in case you want to minimize its effects.
Other scopes help deal with parallax by having an adjustable objective on the scope. This moves the target and the reticle into the same or similar focal plane which prevents them from appearing misaligned. This is done by dialing in the appropriate range in the objective and allows for a larger margin of error when getting behind the optic.
Another thing to keep in mind is that parallax can be an issue when dealing with a red dot and magnifier. While most red dot sights do not have a noticeable amount of parallax, adding magnification to it can produce an increased amount of parallax. This is one of the reasons why holographic sights are often easier to magnify than red dots.