July 01th, 2021
Evelyn: Stephen, many beginners like me are suffered with scope parallax, shall we just choose a red dot sight for shooting? I heard all red dot sights are parallax free!
Stephen: There is a prerequisite for red dot sight’s parallax free, it’s distance. That’s why almost all the red dot sights will show the parallax setting distance in the specification.
Not only that, the position of our eyes, lens quality, etc. will also affect the parallax, but of course the distance is the main factor.
Evelyn: How will the distance influence the parallax?
Stephen: Let me show you the principle first.
Stephen: As you can see in Picture one. There is a built-in light source in the red dot. The light emitted from the light source is reflected by the lens and eventually enters our eyes. But the red dot shown on the lens is actually focused at a very long range. Simply put, the red dot is not focused on the lens, but on a very distant plane.
The distance of this plane is the parallax setting distance which is always set as 50 yards or 100 yards when producing.
Evelyn: Does it mean the red dot sight is really parallax free at this exact distance？
Stephen: No, parallax still exists, but the smallest at this distance, within ±2 MOA for the normal red dot and ±1 MOA for the good-quality red dot sight. In daily use, you may find this phenomenon. Although the parallax setting distance is 100 yards, it seems no obvious parallax phenomenon at 70 yards or even 50 yards.
Evelyn: Yeah, but why?
Stephen: Last time I explained scope parallax to you (More details can be found in ‘How to understand the parallax phenomenon in your scope?’). As picture 2 shows, when the image and the reticle are on the same plane, there is no parallax.
Evelyn:Yes, I remember that.
Stephen: When it comes to a red dot sight, ‘image’ and ‘reticle’ turns to be ‘target’ and ‘red dot’.
When the red dot is focused on the same plane as the target is, there is no parallax; Conversely, parallax exists.
In fact, it is not parallax free at 50 yards or 70 yards but we can hardly notice the varying of parallax in this distance range.
I see. So in theory, all red dot sights have parallax, right?
Stephen: Yes, usually, the more the difference from the set distance, the more parallax can be observed. While there is another kind of parallax on red dot sights which is image shift.
Evelyn: What is image shift?
Stephen: Let me show you. On the right picture in picture 4, the image we see through the red dot cannot be articulated with the background. This is a kind of common image shift of parallax. This phenomenon often occurs on some cheap red dot sights.
Evelyn: I have seen this phenomenon and it turns out to be caused by parallax!
Stephen: What’s more, do you remember that? I once told you when using some poor 1x scope with 2 eyes open, the image of both eyes may not coincide.(More details can be find in ‘Why you need 1x power scope for shooting’)
Evelyn: Yes, l do. What does this have to do with red dot sights?
Stephen: Due to the image shift, this also happened on some poor red dot sights.(Picture 5)
Evelyn: Ok, I got it. Thank you for your sharing, Stephen!
Stephen: You are welcome!
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