Eliminating Parallax

[zukiphile]

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.

 

[Bart B.]

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.

 

[JohnKSa]

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.

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?

 

[Bart B.]

"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:

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.

 

[BBarn]

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.

 

[Bart B.]

Semantics are muddying the waters we're wading in.

Any 3 of these 4 (eye, rear sight, front sight, target aimpoint) not physically aligned on one axis equals parallax.

All four(and the trajectally pointed barrel) must be aligned on one axis if you want to hit point of aim with the bullet.

 

[zukiphile]

Bart, I don't think mere semantics are the source of the confusion here.

Any 3 of these 4 (eye, rear sight, front sight, target aimpoint) not physically aligned on one axis equals parallax.

A misalignment of the latter three does not describe parallax, only a misalignment of the sights. Line a rifle up on target and move the muzzle of the rifle an inch left, but don't move your head. The sights now are not aligned on the target, but that's mere misalignment, not parallax error.

Parallax is

the effect whereby the position or direction of an object appears to differ when viewed from different positions

https://www.lexico.com/en/definition/parallax

Only a viewer can view these objects from different positions. When a viewer sees the same rifle in the same position and moves his head, he views from different positions. A parallax adjustment in an optic eliminates or reduces the change in sight picture (parallax error) at specific distances and caused by the viewer's head movements.

Iron sights don't limit parallax error caused by head movement (movement of the viewer), but limit the head movement possible while still maintaining a correct sight picture.

 

[Bart B.]

Whatever ...... ..... .... ... .. .

 

[JohnKSa]

"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.

I agree with that. However, if there is NOT a straight line from the eye through the rear sight to the front sight to the target, then what is the aim point?

When the eye is not aligned with the sights, then they do not form a straight line.

That's what I'm saying when I say that if the sights are not aligned with each other and with the eye, then there isn't an aim point.

If the misalignment is SMALL, then there is an aimpoint, but it's impossible to say if that misalignment is due to the eye being out of line with the sights (which might be loosely interpreted as parallax) or the sights being out of line with each other because the effect is exactly the same in either case.

A point on the target upon which the firearm is aimed.

Agree. But if the eye is not aligned with the sights and the sights aren't aligned with each other then I would say the gun is not aimed at anything. Right now there is a gun in my safe and although one could draw a line through the sights, it wouldn't go through my eye or to any target--it's not aimed at anything in any reasonable sense of the word.

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

Agree. And from that definition, it follows that if the sights aren't aligned then there's no aim point.

The visual image observed by the shooter when the firearm sights are properly aligned on the point-of-aim.

Again, I agree. But what is it called when the sights aren't properly aligned?

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.

I've never seen any tendency for the eye to "automatically" align anything in the notch of a rear sight.

However, I have noticed some tendency for the eye to try to center things in a well-designed aperture sight. Conversely, I've seen poorly sight systems where the eye tries to center the wrong thing in the rear aperture. One example of this is putting a rear aperture sight on a rifle that has a protective hood that isn't round over the front sight. The non-round hood tends to interfere with the eye's attempt to center the bead in the aperture, and removing the hood tends to improve on-target results.

Any 3 of these 4 (eye, rear sight, front sight, target aimpoint) not physically aligned on one axis equals parallax.

I disagree. No definition of parallax I've seen or that you've quoted says anything like that.

Look, I think you have a point to this thread and the questions you asked.

Maybe instead of arguing about why it does or doesn't make sense to try to incorporate sight misalignment into the definition of parallax we could skip to the point of why you started the discussion.

 

[Bart B.]

I'll try to simplify my point.

After the rear sight is zeroed for target range with the LOS from eye through rear and front sight centers to point of aim on target, for every MOA the LOS is off center in the rear sight thereafter the shot will go one MOA in that direction because the LOF moved that much relative to the LOS. Field of view through the rear sight aperture can be a few to several dozen MOA depending on aperture & eye iris diameters and eye relief from aperture.

Easy to see if you can shoot with aperture sights under MOA for a couple dozen consecutive shots. 1 MOA in the rear sight aperture is sight radius divided by 3600.

 

[Aguila Blanca]

You are not describing parallax.

Parallax is optical distortion. Parallax applies only to scopes, not to open sights or single-plane optics. Parallax error is when the shooter has the crosshairs centered on the target POA, but the image is not correctly aligned with the axis of the bore due to the shooter's eye being off the axis of the scope. For any scope, parallax error does not apply at a specified distance, and does apply at other distances. Scopes that allow adjustment of the shooting distance can be adjusted to eliminate parallax error at multiple distances. (So-called "AO" -- for Adjustable Objective) scopes.)

 

[Bart B.]

Aguila,

A simple everyday example of parallax can be seen in the dashboard of motor vehicles that use a needle-style speedometer gauge. When viewed from directly in front, the speed may show exactly 60; but when viewed from the passenger seat the needle may appear to show a slightly different speed, 55 for example, due to the angle of viewing. No optics nor lenses whatsoever.

None of the dictionaries listing "parallax" state it's limited to optics and lenses.

No scope with the range focus knob opposite the windage knob have a user adjustable objective lens; it's fixed in place.

When the shooter has the crosshairs centered on the target POA, the image is never aligned with the axis of the bore.

 

[zukiphile]

None of the dictionaries listing "parallax" state it's limited to optics and lenses.

Parallax is a phenomenon present whenever a viewer watches two objects at different distances and changes his position. Every child who looked out a car window at plowed fields has seen it.

While the phenomenon is present in many situations, it is a parallax error only where an optic designed to present a target and crosshair on the same plane fails to actually present them on the same plane.

Misalignment of iron sights doesn't give rise to this error because those systems don't present a viewer with target, front post and rear aperture on a single plane, ever. Therefore, that misalignment is not a parallax error.

You are absolutely correct that an alignment error will cause a round to land at a different spot relative to the front post, but since it doesn't occur where the sight picture is correct, the error isn't the result of parallax.

 

[Bart B.]

While the phenomenon is present in many situations, it is a parallax error only where an optic designed to present a target and crosshair on the same plane fails to actually present them on the same plane.

Misalignment of iron sights doesn't give rise to this error because those systems don't present a viewer with target, front post and rear aperture on a single plane, ever. Therefore, that misalignment is not a parallax error.

Yes, they do present a viewer with target, front post and rear aperture images on a single plane . On the retina of the aiming eye. Lateral displacement of each is easily seen.

The eye retina acts like the second image plane in a scope sight where the first focal plane image and reticle are focused.

 

[zukiphile]

Yes, they do present a viewer with target, front post and rear aperture on a single plane . On the retina of the aiming eye. Lateral displacement of each is easily seen.

Emphasis added.

Let's unpack that. The rear sight is just an inch or two from the eye. That's the first focal distance for the viewer, but we don't really focus on it. The front post will be a couple of feet away. That's another focal distance, and the element on which we are supposed to focus. The target will be 25, 50, 100, 200 etc yards away. That's a third focal distance even though we aren't supposed to focus on it. Iron sights do not present a viewer with rear sight, front post and target on a single plane, but three distances only one of which we hold in focus.

In contrast, a scope may present the crosshairs and target on a single focal plane that is within a foot of the eye.

That poor alignment of iron sights can be spotted is true, and demonstrates why that condition isn't parallax error, which will not be easily spotted in a scope unless the viewer moves his head.


If your point is that the best accuracy requires a consistent hold and head position for both a scope and iron sights, I doubt anyone would argue the point.

 

[Bart B.]

I don't think the tiny out of perfect focus distance each iron sight system part puts on the eye retina is enough to matter. My concern is their misalignment at right angles to the LOS. Human eye lens focal length is too short to matter for me.j

Interesting article on aperture sight parallax:

https://www.google.com/search?q=Par...droid-samsung&sourceid=chrome-mobile&ie=UTF-8

 

[JohnKSa]

Here's the link to the article:
http://dougkerr.net/Pumpkin/articles/Aperture_Sight.pdf

After the rear sight is zeroed for target range with the LOS from eye through rear and front sight centers to point of aim on target, for every MOA the LOS is off center in the rear sight thereafter the shot will go one MOA in that direction because the LOF moved that much relative to the LOS. Field of view through the rear sight aperture can be a few to several dozen MOA depending on aperture & eye iris diameters and eye relief from aperture.

When you say LOS, are you talking about moving the eye? Because if one aligns the sights on the target properly and only moves the eye, there will be no change on the target unless the gun is moved too.

If you mean that moving the eye off center from the rear sight and then lining up the front sight on the target (but still keeping it within the rear sight aperture from the shooter's perspective) then the article you linked to states, and provides test data and rationale to support the assertion, that such a movement will have no significant effect on the point of impact.

From the article:
"The second target shows the result for a group fired with the front sight intentionally offset towards the upper edge of the aperture. The “ears” of the front sight were positioned so that they were near the edge of the aperture. This magnitude of offset constitutes a “gross misalignment” of the sights in terms of the standard method of aiming, and under the “common wisdom” would be expected to result in shots that would be displaced from the intended point of aim by a considerable amount because of parallax. However, as can be seen from the target, this “misalignment” of the front sight in the aperture did not result in a significant shift of the bullet group on the target. "

 

[Bart B.]

Yes, the article I linked to states, and provides test data and rationale to support the assertion, that "such a movement will have no significant effect on the point of impact."

How much does the LOF to LOS angle have to be before it becomes significant?

What's the angle between "zero" and "no significant" effect.

It's easy to see a half MOA parallax error in a magnified image in a scope sight. The visual amount seen by the eye with a 10X scope is ten times as big as seen by an unaided eye. Human eye angular resolution varies from 1/2 to 1 arc minute. With a 10X scope, the human eye can see 1/20th to 1/10th MOA parallax error between LOS and LOF. Ten times those values with metallic sights.

If one can really put the line of fire precisely to the same point on target with metallic sights for every shot fired, why do benchresters magnify the sight picture a few dozen times with a telescope?

 

[zukiphile]

Interesting article on aperture sight parallax:

It prompted me to check this at 20 yards (the sun wasn't up and I had to do this indoors). With the small aperture I detected no shift, but with the large aperture, the shift with head movement was considerable.

The authors did a good job of describing how the smaller aperture determines the alignment of the image the eye will pick up. This explains why my accidentally using the large aperture for sighting in resulted in a different point of impact last week even though the group was a normal (for me) size. The small aperture may have helped conceal from me an error in my eye's alignment that the larger aperture didn't conceal.

Even if I had perfect alignment, I would still need the smaller aperture for the better depth of field.

It is an interesting article.

 

[JohnKSa]

How much does the LOF to LOS angle have to be before it becomes significant?

1. You never explained what you meant by your initial statement above LOF/LOS.

2. Using conventional definitions for LOF and LOS, any angle between them should be detectable on the target as long as it's large enough to be detectable on the target.

3. Neglecting LOF and LOS and just talking about the article and using the terminology it uses, according to the article, apparent misalignment between the sights can be expected to have a different effect on target with open sights than it does with aperture sights--both, of course, being metallic sights.

Any sight misalignment in open sights should have an immediate effect on the target, one that can be calculated--but sight misalignment with a typical metallic sight system is not really parallax, as discussed in depth above. Things might be different with aperture sights, according to the article--possibly because a sufficiently small aperture acts something like a lens.

And, of course, things are different yet when discussing optical sights where parallax in the conventional sense obviously comes into play.

I know you're trying to get at something, but your unconventional definitions for the terms you are using are making it really difficult to even understand your questions. let alone answer them in a way that might lead to learning. If the goal is to help people understand things that they currently don't understand, the first step is communication and that means using conventional meanings for the important terms in the discussion.