Zeroing and bench lens distortion

[medmo]

Interesting. Recently, when dialing in a Holosun 407k on a SA Hellcat, I noticed two different groups at 12 yards when using the center of the lens versus the upper 1/3 for dot placement. Not sure if anyone else has seen the linked article below written by Ryder Crockett at greeneyetactical.com. It has some fascinating information, scientifical methods, references things like Pythagorean Theorems, Euclidean Distances which kind of makes my head hurt making me want to take a break and lick a few windows. I've read through it a few times. Great stuff.

https://www.greeneyetactical.com/201...ight-parallax/

[rawkguitarist]

Scott Jedlinski advocates zeroing unbraced and at your normal firing grip to align the dot to the target with your eye how you actually shoot. That's shots touching in a 1 inch square at 10 yards, confirming at 25. Before I get flamed, I'm just relaying one persons method and reason. Although it seems to make sense to me and it's how I've gotten decent zeros. I don't have a good enough bench setup to test it against a traditional 25 yard zero.

[Archer1440]

Scott Jedlinski advocates zeroing unbraced and at your normal firing grip to align the dot to the target with your eye how you actually shoot. That's shots touching in a 1 inch square at 10 yards, confirming at 25. Before I get flamed, I'm just relaying one persons method and reason. Although it seems to make sense to me and it's how I've gotten decent zeros. I don't have a good enough bench setup to test it against a traditional 25 yard zero.

I absolutely agree.

What I generally find is that you can have two different ammunitions deviate considerably in POI at 25 (obvious), yet have very similar grouping at 10 (somewhat less obvious).

That is the main reason why I feel a 25 offhand group is the deciding factor, for me. It's also why when setting a zero for a particular ammo, I take the time to figure out either the needed adjustment, or the hold offset, for other ammo I might shoot (as in using my carry gun with 147 +P for a high round count match with standard 124)

I'm pretty sure Scott covers this fact in his lectures.

(The first marked group on the 25 yard 147+P load was from a zero that was dead center offhand with standard pressure 124)

[Molon]

OP, It’s called parallax error.


Observations On The Effect Of Parallax Error
When Shooting With an Aimpoint Comp M5, a Trjicon MRO and an Aimpoint T2

https://pistol-forum.com/showthread....mpoint-Comp-M5



3-shot groups mean nothing with regard to determining the radial dispersion of a barrel/ammunition combination or a valid center of impact.
The 10-shot group pictured below was fired from a Smith & Wesson MP9 with an APEX barrel at a distance of 25 yards. If I just cherry-picked some 3-shot groups out of those 10-shots I’d have different central points of impact and I could claim that my gun and ammunition were much more accurate /precise than they actually are.






Here’s another example demonstrating that 3-shot groups do not provide a valid indicator of the radial dispersion of a rifle/ammunition combination or a valid center of impact. The test vehicle for this demonstration was a one of my AR-15s that has a free-floated Colt HBAR and is wearing a Leupold 3.5-10X40 LR/T that has ¼ MOA clicks for windage and elevation.







Prior to beginning this demonstration, I fired a 10-shot control group at a distance of 50 yards using a hand-load topped with the Sierra 77 grain MatchKing. The target used is an NRA High Power type target reduced for 50 yards. The X-ring measures 0.75" and the 10-ring measures 1.75". The 10-shot control group had an extreme spread of 0.493”.








Next, I fired a couple of 3-shot groups from 50 yards using Federal XM193 ammunition. (Shooting was conducted at an indoor range, so wind was not a variable in this case.) The two targets are pictured below.







The center of the first 3-shot group was located approximately ¼ MOA high and ½ MOA to the right of the center of the target, which was the point-of-aim. However, the center of the second 3-shot group was located approximately 1.5 MOA low and ½ MOA to the left of the center of the target. The centers of just these two, 3-shot groups are almost 2 MOA apart in elevation.

The centers of the individual 3-shot groups are indicated below by the blue crosses.








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[JCN]

3-shot groups mean nothing with regard to determining the radial dispersion of a barrel/ammunition combination or a valid center of impact.

“Nothing” is hyperbole like clear gel results are worth “nothing.”

3-shot group as a pilot small sample size group with a wider confidence interval, sure.

The less shots, the less confidence interval and spread. People don’t usually report their spreads with a confidence interval, but that’s probably the most appropriate description of it.

3 shot group gives a wider confidence interval, just like any small statistical sample.

It’s worth what it’s worth.

More robust sample numbers and more accurate hardware and less human variable, the smaller the sample size needs to be for a certain amount of confidence.

I liked the way your article really also supported how different the amount of shift could be with different manufacturers.

[JCN]

Regarding parallax… I’m not as educated as many here.

My best understanding is that there are different types of parallax and one type of parallax is described as “image shift” which is why I was trying to describe the more specific phenomenon?

Is that wrong?

What I’m seeing in the cheaper sights is a warping and distortion that isn’t progressive with distance from center.

Like with some poorly installed windshields installed under uneven tension and twisting.

[Molon]

“Nothing” is hyperbole like clear gel results are worth “nothing.”

3-shot group as a pilot small sample size group with a wider confidence interval, sure.

The less shots, the less confidence interval and spread. People don’t usually report their spreads with a confidence interval, but that’s probably the most appropriate description of it.

3 shot group gives a wider confidence interval, just like any small statistical sample.
.

A cherry-picked 3-shot group means nothing. The confidence intervals of the extreme spreads of several 3-shot groups mean nothing as they are a one-dimensional metric that fails to account for the shifting central points of impact of those groups. Radial dispersion is a two-dimensional issue.

The confidence intervals of the extreme spreads of several 3-shot groups do nothing to establish a valid central point of impact from which accurate sight adjustments can be made and they fail to capture a valid radial dispersion for which accuracy/precision claims can be made.


The data in the graphic below was obtained from actual live-fire testing with an AR-15 from a distance of 100 yards.






The extreme spread of the largest 3-shot group (group number 8) is five times as large as the smallest 3-shot group (group number 2). The centers of the various 3-shot groups shifted quite a bit in their location from the point-of-aim. For example, all three shots from group number 3 where higher than the point-of-aim, while all three shots from group number 4 where lower than the point-of-aim. Also, the center of group number 3 was to the left of the point-of-aim, while the center of group number 4 was to the right of the point-of-aim.

Over-laying those ten 3-shot groups on each other with respect to the aiming point forms a 30-shot composite group that provides a valid indication of the central point of impact and as well as a valid indication of the radial dispersion.






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[Molon]

Regarding parallax… I’m not as educated as many here. My best understanding is that there are different types of parallax and one type of parallax is described as “image shift” which is why I was trying to describe the more specific phenomenon?

The image of the target is not what shifts with parallax. Refraction, such as from barrel mirage is something that can cause the image of the target to shift.





With parallax, the position of the red dot with respect to the image of the target shifts with different viewing angles for optics that are not parallax-free.


...

[JCN]

@Molon

I’ll have to put this in more detail later. But I’m trying to describe something different.

I'm going to ask for your help to use the correct terms to define what I'm noticing.

If I had a parallax free system... but a poorly constructed lens of non-uniform thickness / density...

The amount of parallax could be variable, no?

Help me describe this, I think it's a problem with the quality and construction of the lens system more than the theoretical design.

1. The circle dot reticle of the EPS Carry I have is a circle when viewed in the lower 1/3 of the window.
2. That circle is no longer a circle when I move it slightly upwards. It's dented.
3. When I move that circle up a little farther, it gets more circular.
4. When I move it up further... it's dented.

So to me that is a warped lens that adds inconsistency in aiming in the normal center region of the window that I would use in active targeting at speed.

Is it parallax, image shift or something different? I think it's something in the quality rather than the design of the system.

Also, the construction of most PMO has some element of image shift / refraction when you move it in your field of view. There are subtle distortions when viewing the target even with dot off, but with action pistol type use, it's "usually good enough" but lens quality and construction still play a part... it's something I didn't quite appreciate with some of these PMOs.

[JCN]

In anyone with an EPS Carry MRS seeing some severe optical distortion if the reticle isn’t centered in the glass? The reticle on mine turns into an origami figure compared to a 507k reticle which looks the same no matter where it is in the glass. Just checking if this is common, or an RMA issue. Thank you

This is what I’m talking about above. I’m seeing distortion with this optic that I’ve never seen before.

I wouldn’t have noticed it without the MRS… but the single dot people could be affected without them knowing.

If the optic isn’t in a non-distorted place it really affects where the bullets wind up.

@Molon it’s not an issue with the gun accuracy, it’s an optic issue.