![]() That’s not to say you don’t need to focus the image of the tree to make it clear, but do you see how there can be no relative movement when all images are presented on exactly the same plane? Well, that is your digital NV screen with no parallax possible. Now move around and both the tree and menu display (reticle) stay absolutely stationary to each other. Now imagine looking at a TV screen with a picture of a tree on it and pressing the menu button or some other function that brings up a transparent icon (reticle) on the screen. Flip that the other way and if your optic is not parallax free and your eye isn’t in the correct position, you are possibly going to move the rifle to ensure that crosshair is on target, when in fact you are now making the parallax error and the rifle is effectively pointing in a different place. When you translate this to a rifle, even though the scope and rifle are stationary and the crosshair looks like it is on target, the reticle would move off target if a parallax error is present in the optic and your head moves. If there was a crosshair painted on the window representing the reticle, you now have three planes parallax focuses the tree on the window from the front and, the fast focus eyepiece similarly focuses the eye to the ‘window pane’ from the back to remove all relative movement if your eye moves. If you move side to side, the tree and object will appear to move relative to each other – that is a parallax error because the relative distance to each from your eye is different. Say you can see 100m through that window to a tree and there is a small object, say a flower, close to the glass, on the inner windowsill. The most simplistic way to think of this is using the view from your window. Within the optic, there is therefore potential for relative movement if it’s not precisely focused at each end –objective and ocular must be focused precisely on exactly the same point within the tube. This differs from a daylight optic where the image travells through the scope’s optics and reticle. Both are precisely on the same image plane. The digitally added reticle is also positioned and displayed on this screen so there can be no relative movement of the reticle versus the image, full stop. If we are talking digital night vision or thermal, there is, in fact, no parallax error possible as we know it in daylight optics for one simple reason: a camera in the front of the scope views an image on its sensor, which is displayed on the internal screen. Part of the issue is due to some scope designs having large gaps between the scope centre and bore, as people can blame their scope for ‘zero wander’ when in fact it’s non-consistent cheek weld, lack of DOPE (ballistic data on your previous target engagements) and inaccurate range assessment that is the problem. I am often asked whether the way that digital night vision presents the reticle is responsible for increasing parallax error in comparison to ‘normal’ optics – thereby making the importance of a decent, repeatable cheek weld even more important. Chris Parkin explains the finer points of parallax - what it is and what causes it!
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