Sight Alignment Heresy

[ks_shooter]

I have heard and read some version of the following statements many, many times:

"To properly aim aperture sights the front sight post must be centered within the rear aperture."

"If the front post is not centered within the aperture this indicates a sight misalignment and the Point of Impact will be shifted."

"Sight alignment is just as (or more) important than sight picture".​

A friend and I have conducted testing and analysis that indicates that these statements are incorrect when the aperture involved is relatively small (say less than 0.100"). In this case, the aperture of the rear sight is smaller than the pupil of the eye and what is typically observed as sight misalignment does not change the Point of Impact of the bullet. In effect, these types of aperture sights are "parallax free" (within generous limits), similar to a Aim Point sight, and it is sufficient to align the front sight and target; with only a passing concern for centering both within the aperture.

For "ghost ring" style aperture sights this positive affect does not occur since these apertures are typically larger than the pupil. Even so, simple geometry reveals that large amounts of sight misalignment still produce relatively small amounts of shift in POI when compared to standard open sights.

If you use aperture or "peep" sights on any of your rifles you might find the paper that we have written interesting:

http://doug.kerr.home.att.net/pumpkin/#ApertureSight

Please take the time to look at the paper before deciding that we must be wrong. Enjoy.

 

[coelacanth]

glad to hear somebody studied this. . . . .

I've noticed that on my old military rifles ( specifically SMLE no.4 ) I can be pretty cavalier about where the front sight post is in the aperture and still get respectable accuracy out to about 400 yards. This first came to light when some friends and I were shooting timed fire drills to see if we could match the old British army standard and still hit the broad side of a barn. We're not talking match grade accuracy here even from the best of these weapons but it always seemed odd to me that the alignment of the front sight post wasn't more critical. Thanks for the post and the link.

 

[ks_shooter]

I guess no one was in the mood to burn a heretic, or everybody thinks we were right , or maybe our explanation was too obscure.

I offer the following simple experiment that can be done to observe the lack of error when aiming with aperture sights that are "misaligned":

1) Place a rifle with suitable aperture sights (aperture less than 0.100") on a stable shooting rest.

2) Adjust the position of the rifle so that the front sight is aligned with the target, but do so with the front sight offset from the center of the aperture a generous amount, making sure that the front sight is still clearly visible (not touching the "fuzzy edge" of the aperture).

3) Without moving the rifle, adjust your head position so that the front sight appears centered in the aperture.

4) Observe the alignment of the target and front sight.

If the final "centered up" sight picture has the front sight and target properly aligned, then it can be concluded that the position of the front sight within the aperture does not effect the aiming process. The initial misalignment of the sights did not cause any parallax shift between the front sight and target, so the resulting "aim" was still correct.

With open sights, if you misalign the sights, align the front sight to the target, and then (with out moving the rifle) re-locate your eye so that the front and rear sights once again appear aligned, you will notice that the front sight and target are no longer aligned. The parallax from the misaligned sights caused the initial aim to be in error. With (small) aperture sights this does not occur.

 

[Dr. Dickie]

For "ghost ring" style aperture sights this positive affect does not occur since these apertures are typically larger than the pupil. Even so, simple geometry reveals that large amounts of sight misalignment still produce relatively small amounts of shift in POI when compared to standard open sights.

My understanding is, the whole point of a "ghost ring" sight is that you naturally center (well enough) the front sight post in the ring, so you can effectively ignore the ring (let it go ghost).

 

[ks_shooter]

Dr. Dickie,

You are correct. There isn't really any difference in how target apertures and ghost rings are used. It just a matter of degree. The ghost ring has a very large field of view and allows for faster target acquisition when compared to a smaller aperture. The penalty is that it is harder to properly "register" the front sight within the aperture. Centering maybe natural but the overall accuracy of the centering ability is worse with a larger aperture.

In the case of ghost ring apertures the aperture diameter is larger than the pupil of the eye and the advantage of parallax suppression is not obtained. With the larger aperture sight misalignment WILL translate into a change in POI. The amount of shift is still surprising small, though.

Assume a ghost ring aperture that is 0.250" in diameter, and a sight radius of 20". Also assume that the user is able to center the front sight somewhere in the middle half of the aperture field of view. This is fairly "sloppy" sight alignment, but could result during rapid target engagement.

This amount of sight misalignment would cause the POI to shift by an angle of arctan((0.250/4)/20)=0.179 degrees or 10.7moa. This is a large amount of error for target shooting, but would be acceptable for combat under 50 yards, which is what ghost rings are made for.

 

[BigG]

You may be right, but if so you might be sacrificing the times you could even see through the peep sight. Low light would probably make it more difficult.

 

[Kaylee]

hunh... well that's neat. I like seeing conventional wisdom taken out and tested every now and again.

I'm gonna have to try that. Thanks.

 

[ks_shooter]

BigG,

In low-light situations the pupil dilates, so the allowable size of the aperture while still suppressing parallax is increased. The size of the aperture defines a spot of light that falls on the pupil of the eye. As the eye moves around behind the aperture, the positioning of the the spot of light changes on the pupil. As long as this spot of light is contained within the edges of the pupil no error is caused by "sight misalignment".

In normal outdoor light, the pupil is about 2~3 mm in diameter. Since a typical target aperture is about 1 mm, there is a lot of room, so to speak, to move around before the 1 mm spot of light reaches the edge of the pupil. The small aperture on an M-16 is about 2 mm, so there is less wiggle room before error occurs. The large aperture on the M-16 is about 5 mm in diameter. This will cause errors from sight misalignment since the pupil is now smaller than the aperture.

If the eye is dark-adapted, though, its diameter can grow as large as 8 mm. In this case, the larger aperture would no longer suffer aiming errors from sight misalignment.

 

[Red Tornado]

Excellent article. I'm going to break out the M1 next chance I get and give this a try.
RT

 

[ks_shooter]

I am re-posting the link to the paper that Doug and I wrote. The paper has been updated to include tests that use a camera to investigate the phenomenon. Doug performed all of the setup and testing, and the results conclusively demonstrate that the effect of parallax suppression is real. It also provides a convenient set of images to illustrate the phenomenon for the reader. The new information is in the section titled “Optical Model Tests”:

http://doug.kerr.home.att.net/pumpkin/#ApertureSight

A detailed explanation of the methods and procedures used in the camera testing are contained in a companion paper:

http://doug.kerr.home.att.net/pumpkin/#ApertureSightDemo

 

[Caimlas]

Incredible! Thanks a bunch. I'm going to repost this; this is good info.

 

[Red Tornado]

The article was even better with pictures, thank you.

I have a (possibly silly) question. Do your findings suggest that the parallax suppression will also affect the vertical centering of the post in the aperture? That is, if my eye drops by 1.25mm, thereby raising the post in the sight picture, will it still remain on target? Or is this completely irrelevant to the research and testing you've done?

Thanks,
RT

 

[Vern Humphrey]

it is sufficient to align the front sight and target; with only a passing concern for centering both within the aperture.

That is the proper way to use aperture sights -- there is no need to consciously center the front sight in the aperture. It happens automatically.

 

[ks_shooter]

Red Tornado,

Parallax suppression acts equally in all directions. We only used lateral offsets in the camera test for convenience and brevity. In my life fire experiment I tested with an upward offset of the front sight and target and did not change the position of the group.

 

[Red Tornado]

Thanks, that's what I thought, but I didn't want to make assumptions. This knowledge should really be of assistance when trying rapid fire.

I read some of Doug's articles, and only have one question. How freakin' smart it he?
RT

 

[ks_shooter]

That is the proper way to use aperture sights -- there is no need to consciously center the front sight in the aperture. It happens automatically.

Vern,

I agree with the first part of your statement, and mostly with the last part. I would state it slightly differently, though:

"The eye can naturally center the front sight with enough accuracy so that no aiming error occurs."

This emphasizes the finding of our paper that precision alignment of the front sight in the center of the aperture is not needed. What is needed, is a "coarse" centering of the front sight.

Our testing seems to indicate that with typical target apertures sizes, the "sweet spot" where parallax suppression occurs is about the middle center of the aperture. The amount of attention it takes to accomplish this manner of centering is minimal, and can be done almost subconsciously.

As the pupil constricts, or the aperture is made larger, the "sweet spot" shrinks. When the pupil and the aperture are the same size parallax suppression is no longer present, only parallax reduction, and some aiming error starts to occur.

To say that the eye will automatically center the front sight can be interpretted to imply that precise centering is required, which is not the case. I believe that the origin of the often cited admonition about "natural centering" is not due to some uncanny abiltiy of the eye, but rather to the fact that precision centering is not required, and therefore little effort is required to obtain sufficient results.

When I first started shooting Highpower I was nagged by a feeling that my sight alignment was inadequate and that it was the cause of the occassional flyer. I knew that my ability to center the front sight wasn't as accurate as my ability to align the front sight and the target. But, I was being told two things:

1) The front sight needs to be centered in the aperture just as carefully as the front sight and target are aligned (some people even claimed that sight alignment was MORE important than sight picture)

2) Don't worry about it because your eye will handle it.

I never bought into this, and now I know that these statements are false. The end result may be the same: don't worry too much about centering the post, but with a proper understanding of how the aperture sight works, nagging doubts about whether sight alignment is adequate or not can be dismissed.

 

[ks_shooter]

Red Tornado,

Rapid fire is a primary application of this theory. When time counts you don't want to waste time doing something that isn't going to make any difference. During slow fire, there is time to dress up sight alignment if you want to, but during a long match this will contribute to more eye and mental fatigue.

In discussing this with a smallbore shooter he indicated that even if the sight alignment wasn't wrong, that the small head movement indicated my the front sight not being centered could still affect shot placement. This could be true in olympic class shooting, but would be a secondary and not a primary error source.


P.S. - Yes, Doug is one smart cookie. He and I have exchanged about 100 emails during the writing of these papers, although we have never met.

 

[ks_shooter]

While discussing this topic on another forum, a member raised the question of why sight adjustments work if eye movement doesn't cause a shift in the Point of Aim. After some thought I gave the following reply, which I think adds another piece to the puzzle:

"In thinking about your question again this morning I had an "epiphany" of sorts. It occurred to me that the fact that 1/4 minute sight adjustments actually work is strong evidence that sight alignment doesn't matter. For a 1/4 minute click to move the Point of Impact a true 1/4 minute on the target (assuming the conventional wisdom about the need for sight alignment is true) the sight alignment would have to be consistent to with a fraction of a 1/4 minute. Otherwise sight adjustments would get swamped out by variations in sight alignment.

To make a 1/4 minute sight adjustment work accurately, sight alignment would have to be maintained to a level of perhaps 1/10 of a minute or less. I don't think anyone can reliably center the front sight within 1/10 of a minute from shot to shot. The typical field of view through a rear sight is something like 100 MOA. To center the front sight to within 1/10 of minute would require a placement precision of 0.1% within in the aperture. This could also be thought of as requiring the front sight to be centered to within 1/60 of its width. I don't think this is possible by mortal man."

 

[Sunray]

If you're thinking about or concentrating on the rear sight at all, you're using it incorrectly. You look through an aperture sight at the front sight. Nothing more.

 

[ks_shooter]

Sunray,

I agree. But this is not how Marksmanship is currently being taught, and that is what I am attempting to change. The emphasis on the need to center the front sight in the aperture is universal in the sources (including Military training manuals) that I have looked at.

Here is an example from the Army Field Manual on Rifle Marksmanship (FM 23-9):

(1) Rifle Sight Alignment. Alignment of the rifle with the target is critical. It involves placing the tip of the front sight post in the center of the rear sight aperture (Figure 4-16). Any alignment error between the front and rear sights repeats itself for every 1/2 meter the bullet travels. For example, at the 25-meter line, any error in rifle alignment is multiplied 50 times. If the bullet is misaligned by 1/10 inch, it causes a target at 300 meters to be missed by 5 feet.

Emphasis added is mine. This teaches the trainee that he must use as much care in aligning the front sight in the aperture as he should in aligning the front sight with the target. I believe this is a fundamentally flawed understanding of how aiming works with an aperture sight.

Here is a picture taken from the Army Sniper Training Field Manaul (FM 23-10) that purports to show the shift in POI due to sight misalignment.



This information is incorrect (even ignoring the mis-labeling of the scope versus the iron sights). With an aperture sight, and the amount of offset shown in the figure, the POI will follow the tip of the front sight.

 

[ks_shooter]

My comments in Post #18 are incorrect. After thinking about it some more I realize that sight alignment precision would not need to be small compared to the size of the sight adjustments. Sorry for any confusion this generated.