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Old August 11, 2019, 08:49 AM   #1
Bart B.
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Eliminating Parallax

Is parallax eliminated the same way with both metallic and optical sights?
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Old August 11, 2019, 10:12 AM   #2
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Is parallax eliminated by iron sights, or is error just reduced by the size of the aperture?
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Old August 11, 2019, 12:09 PM   #3
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I believe parallax is an issue unique to optics, and does not occur with open or aperature sights that utilize no optical elements.
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Old August 11, 2019, 12:39 PM   #4
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Originally Posted by BBarn View Post
I believe parallax is an issue unique to optics, and does not occur with open or aperature sights that utilize no optical elements.
Can an aperture rear sight with a -.5 diopter corrective lens have parallax problems?

Does a barrel aimed with metallic sights always point to the same place on target regardless of how they may appear relative to each other and the target?

Last edited by Bart B.; August 11, 2019 at 01:38 PM.
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Old August 11, 2019, 03:22 PM   #5
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How are you defining parallax? I don't see how you could have parallax in metallic sights as the requirements for aiming with metallic sights pretty much rules out parallax by forcing the shooter's eye into proper position.
Quote:
Does a barrel aimed with metallic sights always point to the same place on target regardless of how they may appear relative to each other and the target?
If used properly, metallic sights always appear the same relative to each other and the target. I'm not sure what this question means.
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Old August 11, 2019, 04:21 PM   #6
Bart B.
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Parallax with metallic sights exists when front and/or rear sight center is not on the line of sight between eye and target center.

With a good zero for target range, when front and rear sight centers are on the line of sight (LOS) from eye center to target center, shots so fired will strike the target center.

If front sight center is to the right of aperture center but centered on target upon firing, shot will go right because the line of fire (LOF) points right of LOS.

If rear sight center is to the right of front sight centered on target when fired, shot will go left because the LOF points left of LOS.

Shooter has to conciously correct front and rear sight alignmen to the LOS from eye to target.

Last edited by Bart B.; August 11, 2019 at 05:06 PM.
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Old August 11, 2019, 04:32 PM   #7
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If front sight center is to the right of LOS upon firing, shot will go right.

If rear sight center is to the right of LOS when fired, shot will go left.
If either of those are true then the sights are not zeroed. I don't see that as a parallax problem.

Another shooter may hold the rifle differently and a rifle zeroed for me may not impact in the same spot as someone else. And I see this as more of an issue with iron sights than optics.
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Old August 11, 2019, 04:53 PM   #8
Bart B.
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Originally Posted by jmr40 View Post
If either of those are true then the sights are not zeroed. I don't see that as a parallax problem.
Yes, I erred. Corrected the post. Thanks. Sometimes hard for me to explain. It's complicated.
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Old August 11, 2019, 04:55 PM   #9
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Originally Posted by jmr40 View Post
Another shooter may hold the rifle differently and a rifle zeroed for me may not impact in the same spot as someone else. And I see this as more of an issue with iron sights than optics.
It's equal both ways. I've observed this a few times shooting 4 man team matches with the same rifle and ammo.

Last edited by Bart B.; August 11, 2019 at 05:02 PM.
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Old August 11, 2019, 05:07 PM   #10
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Parallax with metallic sights exists when front and/or rear sight center is not on the line of sight between eye and target center.
Strictly speaking, that's sight misalignment.

Parallax exists when the sight(s) appear to be properly aligned with the target but aren't because the shooter's eye is not properly aligned with the sight(s). That can't really happen with metallic sights.
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Last edited by JohnKSa; August 11, 2019 at 05:24 PM. Reason: Corrected wording.
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Old August 11, 2019, 05:43 PM   #11
Bart B.
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Originally Posted by JohnKSa View Post
Strictly speaking, that's sight misalignment.

Parallax exists when the sight(s) appear to be properly aligned with the target but aren't because the shooter's eye is not properly aligned with the sight(s). That can't really happen with metallic sights.
I have demonstrated my reasoning is factual with a rifle hard mounted in place aimed at a target then moving front and rear sights so people can see what happens.

Also with a collimator and live fire exercises.
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Old August 11, 2019, 07:00 PM   #12
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Quote:
Originally Posted by Bart B.
I have demonstrated my reasoning is factual with a rifle hard mounted in place aimed at a target then moving front and rear sights so people can see what happens.
Emphasis added. What you describe above is not what I understand as a parallax problem in an optic.

I know I have a parallax problem in an optic when the optic is zeroed and the rifle stationary as I look through the optic and move my head, but not the rifle or the sights. Parallax will make the crosshairs appear to move on the target even though I know the point of aim can't actually be changing.

If I do the same with an iron sight rifle (move my head while the rifle is stationary), the front post and target move off center, so I no longer have what appears to be an aligned sight. That's not a parallax error because it isn't an error, but the iron sights doing what they are designed to do -- tell you when the back and front are or are not aligned to a spot on the target.

Last edited by zukiphile; August 13, 2019 at 10:13 AM.
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Old August 11, 2019, 07:17 PM   #13
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post #5 and #10.
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Old August 11, 2019, 07:28 PM   #14
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Is parallax eliminated the same way with both metallic and optical sights?
Yes.
The only way to eliminate parallax is to put the muzzle against the target.
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Old August 11, 2019, 08:03 PM   #15
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Parallax in a rifle scope exists when the target image is not in the same plane as the reticle. It can be corrected by refocusing the objective (front) lens so that the target image is corrected to the same plane as the reticle, such as on adjustable objective rifle scopes. Focusing the eyepiece does not correct for parallax, it corrects for refractive errors in the eye.

Since there is no target image produced by an open sight system, there is no parallax. A diopter lens in an aperture sight is used to correct refractive errors in the eye.
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Old August 11, 2019, 08:08 PM   #16
Bart B.
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Quote:
Originally Posted by zukiphile View Post
If I do the same with an iron sight rifle (move my head while the rifle is stationary), the front post and target move off center, so I no longer have what appears to be an aligned sight. That's not a parallax error because it isn't an error, but the iron sights doing what they are designed to do -- tell you when the back and front are or are not aligned to a spot on the target.
That's a parallax situation, not a parallax error. The parallax error is how far bullet misses point of aim when a parallax situation exists.

We first must agree on what parallax is defined as by some standard; perhaps a dictionary?

https://en.wikipedia.org/wiki/Parallax

Last edited by Bart B.; August 11, 2019 at 09:16 PM.
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Old August 11, 2019, 09:57 PM   #17
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I have demonstrated my reasoning is factual with a rifle hard mounted in place aimed at a target then moving front and rear sights so people can see what happens.
I don't disagree with your explanation of what happens when the sights are misaligned as you describe. I just don't think it fits the definition of "parallax".
Quote:
We first must agree on what parallax is defined as by some standard; perhaps a dictionary?
I agree 100% with that definition of parallax.

However, that can't happen with metallic sights because there are TWO components to the sights, not just a single aiming reticle. The eye, the front and rear sight and the target all must be in a line when the rifle is aimed. As soon as the eye moves out of line with the sights, the sights are no longer lined up with each other and so it's obvious that there's a problem.

In a system with true parallax, the reticle/aiming device does not show an obvious problem when the eye moves off axis--it's just that the bullet won't hit where the reticle says it will.
Quote:
The parallax error is how far bullet misses point of aim when a parallax situation exists.
There is no defined "point of aim" when the eye is out of line with metallic sights or when the metallic sights are misaligned with each other. The only way to see the point of aim is for the sights to be aligned with each other and with the eye and there's no parallax when that is the case.

In other words, if we're talking about an apparent movement of the "point of aim" on the target due to the motion of the eye, it can't happen with a metallic sight setup because once the eye moves off axis, there is no longer any definable "point of aim". There's just a multiply misaligned system.

Moving the eye off axis in an optical system that is susceptible to parallax creates a parallax situation where the apparent aiming point is not the same as the actual aiming point.

Moving the eye off axis in a metallic sight system effectively "destroys" the aiming point so there is no longer any apparent aiming point at all.
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Old August 12, 2019, 05:47 AM   #18
Bart B.
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Moving the eye off axis in a metallic sight system effectively "destroys" the aiming point so there is no longer any apparent aiming point at all.
Therefore, it is impossible to call a shot when that happens and the bullet may not hit the 6 foot square target paper 200 yards away. There is no way to tell where the front sight appeared relative to the bullseye when a shot is fired

Right?
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Old August 12, 2019, 06:51 AM   #19
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Quote:
Originally Posted by Bart B.
Therefore, it is impossible to call a shot when that happens and the bullet may not hit the 6 foot square target paper 200 yards away. There is no way to tell where the front sight appeared relative to the bullseye when a shot is fired

Right?
Calling a shot is a matter of prediction and judgement, not a merely physical phenomenon.

While parallax can be observed when any two objects are at different distances, misalignment of iron sights isn't the same phenomenon as parallax evident in an optic at a given distance. The ideal on an optic is to have everything focused on the same plane. Since that can't ever happen on iron sights, parallax can't be eliminated the same way. On one parallax is a simple alignment problem; on the other its a focal plane issue.

One can remedy or reduce parallax error in an optic with the use of an aperture that demand better alignment, but since greater flexibility and ease of use are part of the advantage of an optic, that's not a very popular remedy.
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Old August 12, 2019, 09:04 AM   #20
Bart B.
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Originally Posted by zukiphile View Post
Calling a shot is a matter of prediction and judgement, not a merely physical phenomenon.

While parallax can be observed when any two objects are at different distances, misalignment of iron sights isn't the same phenomenon as parallax evident in an optic at a given distance. The ideal on an optic is to have everything focused on the same plane. Since that can't ever happen on iron sights, parallax can't be eliminated the same way. On one parallax is a simple alignment problem; on the other its a focal plane issue.

One can remedy or reduce parallax error in an optic with the use of an aperture that demand better alignment, but since greater flexibility and ease of use are part of the advantage of an optic, that's not a very popular remedy.
Both iron and scope sights have parallax problems when aim point on target, sight reference points and aiming eye are not on the same axis. Doesn't matter how far apart they are.

I've intentionally misaligned aperture sights to see how far and direction bullets strike from point of aim. I used to think otherwise until my small arms Marksmanship Instructor showed me how to prove otherwise. Easily done if you can shoot your iron sighted rifle sub MOA for 20 shots.

Remember the back end of the barrel moves left if the front sight appears right of aperture center. Both the LOS and LOF pivot on the front sight. That makes the bore point to the right from point of aim on target. Bullet will go that way from point of aim when fired.

Last edited by Bart B.; August 12, 2019 at 09:16 AM.
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Old August 12, 2019, 09:23 AM   #21
zukiphile
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Quote:
Originally Posted by Bart B.
Both iron and scope sights have parallax problems when aim point on target, sight reference points and aiming eye are not on the same axis. Doesn't matter how far apart they are.
When eye, target and iron sights are not on the same axis, they aren't aligned. There isn't a parallax adjustment for that other than aligning them.

The ideal for parallax correction in an optic involves the eye at one end and a single focal plane on which that eye sees the target and crosshair; that can provide a cone rather than a single axis at which the reticle will accurately represent the point of impact. Above, you've described a lack of parallax correction. The reason you can have that specific axis is that the focus of target and crosshairs cross at different distances.

To address your original question, fixing alignment by getting everything on one axis can work on an optic too. I've gotten good results from scopes without parallax correction at shorter distances just using a stock that fits well enough to reduce head movement. It isn't perfect, but it gets rid of the sort of gross errors one can see at shorter distances.

Last edited by zukiphile; August 12, 2019 at 09:46 AM.
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Old August 12, 2019, 12:34 PM   #22
Bart B.
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Why is that knob opposite a scope's windage knob commonly called a "parallax" adjustment when all it does is focus the upside down first image plane on the reticle in the second image plane right side up?

When all scope's front lens cell was twisted to focus the target image upside down on the first focal plane, it was called "range" or "target" focus. Most knew the erector tube lenses refocused and inverted that image on the reticle in the second focal plane. Too bad its well understood name never made it to all scopes with that third knob.
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Old August 13, 2019, 01:06 AM   #23
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Therefore, it is impossible to call a shot when that happens and the bullet may not hit the 6 foot square target paper 200 yards away. There is no way to tell where the front sight appeared relative to the bullseye when a shot is fired.
It's possible to calculate it, or perhaps to make a table of where the bullet will hit based on various levels of sight misalignment. Most people would be totally unable to predict where a shot would hit with any degree of accuracy. They could say it will go farther left than intended, or higher than intended, but it is possible to get more precise if one puts some effort into it, I suppose.

None of that has anything to do with parallax using the definition you chose and linked to.
Quote:
Both iron and scope sights have parallax problems when aim point on target, sight reference points and aiming eye are not on the same axis. Doesn't matter how far apart they are.
With metallic sights, there is no definable aim point when the eye is off axis from the sights. With the sights misaligned there is no aim point. Yes, the bullet will hit somewhere, and with a lot of work a person could figure out where it will hit, but there's no aim point.

When you're talking about aiming with metallic sights, it involves aligning the sights and the eye. With the sights and the eye aligned, the sights create an aim point on the target. When the eye is off axis, there is no aim point because for there to be an aim point, the sights and the eye have to be aligned.

I think what you're trying to say is that because the bullet will hit somewhere even with the sights misaligned that there must be an aim point. That's not true. If you pull the trigger without aiming at all, the bullet still hits somewhere--but there was no aim point.

Or maybe what you're saying is that because it is possible to calculate where the bullet will hit, or predict it with some other methods, even when the sights are misaligned, that there must be an aim point. But that's not true. If the metallic sights are not aligned with each other and with the eye, then there's no aim point.

I suppose if there are SMALL misalignments, then one could argue that there is still an aim point but that it won't correspond with the impact point. The problem is that it's not possible to say if that situation is due to the eye being off axis from the sights or the sights simply being misaligned. In other words, there's still no way to definitively call that parallax because it's impossible to know if the problem is the eye being out of the axis with the sights very slightly, or if the eye is in the right place but the sights are misaligned slightly.

Maybe it would be easier to start out with an answer to this question that we can all agree on.

If metallic sights are not aligned (let's say that the front sight isn't in the notch or aperture or maybe it is even completely obscured by the rear sight) what is the aim point?
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Old August 13, 2019, 06:31 AM   #24
Bart B.
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"Aim point," to me, is the place on target the place on target the front sight is aligned to in the sight picture. It's the far end of the straight line of sight from the aiming eye through the rear sight to the front sight then to some place on target.

Does that make sense? If not, we need to establish a set of names, terms and conventions.

From SAAMI's glossary:
Quote:
AIMING POINT
A point on the target upon which the firearm is aimed.

POINT-OF-AIM
The exact point (at the target) on which the shooter aligns the firearm’s sights.

SIGHT PICTURE
The visual image observed by the shooter when the firearm sights are properly aligned on the point-of-aim.
The aiming eye does not automatically (subconsciously?) align anything to the visual center of the aperture in the rear sight. Nor the notch in an open rear sight. This age-old myth continues to surface in the shooting sports.

Last edited by Bart B.; August 13, 2019 at 07:36 AM.
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Old August 13, 2019, 07:18 AM   #25
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Parallax is the apparent shift in location of one object relative to another object at a different distance by a change in the observer's position. With firearms, parallax can exist with a single sight (such as a shotgun bead), but does not exist with metallic sights consisting of two or more elements. Parallax requires an observer [eye], an single middle object, and a distant object. A pair of metallic sights are aligned in only one eye (observer) position, and movement of the eye position misaligns the sights. So the various metallic sight senarios stated here are examples of sight misalignment as opposed to parallax.
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