The term parallax has many applications across various different fields of study. It is most commonly used in astronomy, photography, and most importantly for us, hunting. In the most general sense, what it means is that if you do not look at something dead straight ahead, you are going to get an incorrect viewing. You have two eyeballs on your head, so you are getting two different images that are being pieced together simultaneously to create images in your brain when you are looking through your scope.
It is an important feature to be aware of because when you are using a crossbow scope you need to be able to perceive depth. Look at the video below before moving on to help you better understand the terms and their application.
Two things affect how much parallax occurs when shooting. The first is how much distance there is between you and the target. The second is the exit pupil size. You will have less instance of parallax if you line up your eye perfectly with the optical axis on your crossbow scope. There is no sure fire way to completely eliminate parallax, but you can still lessen its effects.
Scopes made for long range shooting should have a parallax adjustment. This is because even the slightest deviation could result in a large discrepancy in shot placement. This adjustment knob will help to bring the image into the right focal plane so that you can anticipate the distance.
Be aware of tactical scopes because many of them are not equipped with parallax adjustments. We would also like to make a point to tell you that no amount of correction will help if the scope is of poor quality. Some of the lower quality scopes out there have questionable objective lenses that greatly affect parallax. Before you send out that nasty letter to your scope manufacturer, make sure to check that you have not improperly mounted your scope because if you mount is not secure it will cause fluctuations in parallax.