I have a question about your video:
The parallax (explained at 31:00) is only evident in scopes, not red dot sights, holographic sights, or iron sights?
Simple book definition: Parallax is a displacement in the apparent position of an object viewed along a line of sight.
An optical sight is constructed with a series of lenses. If the reticle and target appear in the same focal plane, a slight shift in your head/eye position will continue having the reticle/target image appearing in the same relationship. If the reticle and target are NOT in the same focal plane, this head/eye movement will give the appearance of the reticle moving in relation to the target even if the rifle/barrel is not moving.
With optical sights, the objective lens focuses the target image into the same plane as the reticle and needs to be checked for each distance shot. Checking for and making this focus adjustment will take out the parallax. The ocular lens focuses the reticle and is adjusted once for the shooter.
Red dot and holographic sights are theoretically parallax free. In practice, red dots may have some parallax inside 50 yards. Keep the dot/reticle centered in the tube and parallax is eliminated.
With iron sights, parallax error is eliminated by focusing on the front sight to keep it centered in the rear sight.
Firearm User Network 
John Tate
Jan 24, 2014 @ 15:24:50
I think the wording of this article, and maybe the video (I don’t have time to watch now.) are in error, at least as far as whether optical or iron sights determine parallax error … as I understand it.
For one thing, parallax error is not dependent upon the viewing mechanism (optics or iron); instead, it is a factor of how far one line of sight is from another. Examples: distance between eyes, distance between lenses on a twin-lens reflex camera, DISTANCE SIGHT IS ABOVE BORE.
Maybe I’m not understanding the content of the message.
By the way, parallax (as I described it) is crucial for depth perception and navigation. My daughter, when a ROTC student doing land navigation, had to have this old sailor show her how to use constructive parallax to find her position based on multiple observations of a distant object taken from multiple positions.
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John M. Buol Jr.
Jan 24, 2014 @ 15:28:13
I admittedly over simplified it. The point I was trying to make while keeping simple is that the reticle/sight won’t properly indicate the true line of bore/lay of the barrel even if the reticle/sight appears in proper relationship to the target if there is parallax error.
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John Tate
Jan 28, 2014 @ 14:48:17
Full disclosure: I teach ‘crime scene photography’
I wasn’t trying to “pick on the author,” certainly not you. But the idea of parallax is central to all optical analysis, especially where refraction is involved. That’s why we must FOCUS … to get all those parallel lines of light focused on the “focal plane.” (Think of photographic issues like “focus,” “optical infinity,” “depth of field” etc.*)
With a red dot or iron sight (or any sighting device/method) which has no refraction, the only parallax related to focus is in your eye. With a magnifying sight, one stage of refraction is added.
My point, parallax will ALWAYS be present, if only in your eye. Because you were segregating magnifying from non-, I thought you meant the parallax I mentioned above. A bit technical, but always there.
My response was addressing the parallax that annoys shooters – the (usually vertical) distance between the line of sight versus line of bore.
I know you’ve familiar with external ballistics programs that have as one input variable the distance of the sight above the bore. That’s to correct for parallax. Also, I bet you’ve seen some silhouette pistol shooters who have their sight mounted 4 ~ 6 inches above the barrel. Lots of wild parallax there.
Stay well,
John Tate
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John M. Buol Jr.
Jan 28, 2014 @ 14:54:14
Thanks for the additional explanation. Adding more good info isn’t picking on anybody, especially if it improves on what was said.
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