A Primer on Parallax at Breach Bang Clear – Nothing is Truly "Parallax Free"

Unfortunately, that last video was not helpful.

The target and dot are not in focus for some of the optics they show. All you can try to do is attempt to mentally track the position of the target.

What then happens is that it seems the red dot is way off, or moving, When in reality, refraction within the sight is causing the target to appear in a different location.

There were times the shift in apparent position was pretty drastic.

If they could not get the shot to focus well, another method of holding the optic steady, and aimed at the target, and then shifting the camera around may work.

I have done similar checks on optics myself, mounting the optic or rifle on a block, aimed at a target at the proper distance, and then shifted my head position usually from a few feet way, and watched the dot.

I have found a faulty optic this way, that got sent back to the manufacturer for replacement. I was having a hard time getting a good 50yd zero, after checking the optic I found this issue. So it turned out that little changes in the alignment of my head, caused big shifts down range.

Edit:
Reading the source article, he addresses these issues.

This article is very poorly written, and the videos don't explain even half the issues.

Firstly, on a TRUE collimating sight the light from the light source is transformed either trough a lense (lense and perfecly flat piece of glass at 45 degrees) or by using a "shortcut" like most if not all red dots for small arms today - by using a curved piece of semi-reflective glass turning the source of the light which radiates in all direction - or specifically in the general direction of the lens/glass "mirror" - and turning the light into almost coherent light. That is the so-called "focused on infinity" concept, but we'll get to that later.

If a lens-and-45-degrees-glass-setup is not used as it rarely is, with the obscure and historically redundant gunsights for some AA guns and almost all WW2 and post-WW2 era aircraft, then the apparent "parallax" issues arise.

This comes from the inherent problem of the light source that has to be at a very specific angle towards the curved reflective glass. Easiest way to see this is with either type of "open" red dot sight like RMR. This curved glass, being at a slight angle towards the source of the light and the shooter, must have very, very precise curvature to give the impression of the "dot" being for example the correct shape, say a small round dot. On piece of s*** red dots like the Redring:
http://www.redringsight.com/

you can see that this problem is amplified immensly, as the light source is not just below the curved glass but at an off-axis angle, so when viewed like in the vide from side to side the shape of the "ring" will change from round to oval (!!!).
Now the same piece of glass has to have an equally opposing (compensating) curvature on the other side so that it does not act as a lens, which is nearly impossible, and if possible it would only happen when the shooter is looking perfectly trough the center of it.

The greatest downside however in the PURELY PERCEIVED parallax in red dots, and not actual parallax, is very simple to explain:

Watch this picture first https://en.wikipedia.org/wi...
now imagine that instead of the lens being "busy" with EXCLUSIVELY transforming that light into almost coherent light, instead the lens and reflective glass is one unit. So when you have a look and your eye is aligned perfectly and sight picture is fine, all seems to be in order. If however you move you eye to either side (at an angle or just side to side, still perfecly parallel to the gunsight) you will eventually observe the target trough a piece of that reflective glass that is curved. Hence the apparent "parallax", though not as such, more like an inherent defect of over-simplification of what a collimating gun sight could/should be. In other words looking trough the edge of what is in effect a lense (thought it was never ment to be, for the shooter's view at least) will absolutely change the sight picture but NOT of the dot, but of the observed target.

Things are a *bit* better with holographic sights, but what happens with the Eotechs and the like is more related to the extreme complexity of how a holographic sight works, and how perfectly aligned and precisely manufactured all components are.
Finally the "parallax" issue, or rather out-of-focus issue, comes from the same piece of curved glass that has to do all these things at once: provide a sufficiently clear field of view (yeah right!), have the perfect, specific curvarture (never possible), be at a very specific distance from the light source (doable, but difficult) and be at the correct angle to compensate for the light source being off-axis (most often below the glass). All in all too many hoops to jump trough for a single component, hence all these issues.

But that being said, since red dots are intended for zero to moderate ranges (100m? 200m?) these problems won't be burning issues, not compared with iron sights at least.

Wow, quite a TL;DR I have written here, he he.
Last point:
while parallax *adjustment* can be possible with actual scopes which is in layman's terms a focusing issue, on red dots this is almost impossible. On a scope the shooter can actually observe what happens with the recticle & target picture (if someone has a S&B scope you know what I mean), on a small no-magnification red dot this HAS to be done from the factory. Firstly because the shooter will have great issues adjusting it even if it is possible (you'll need to immobilize the whole gun), secondly because a the factory they can do it much better, which they don't do as much. Very simple "shortcut" to sort the crappy sights from the better ones is this - if it's a dot, is it perfecty round? Yes - good sight, ellyptical, or whatever else - bas sight.