TargetTalk

When aiming with an aperture sights it is almost always stressed that careful attention must be paid to centering the front sight within the rear aperture. Offsets of the front sight within the aperture are said to constitute a change in sight alignment and will result in a misplaced shot.

I have recently co-authored a paper that investigates this assumption. The conclusion we reached was that "sight alignment" is essentially irrelevant, and that "sight picture" controls the Point of Impact (sight picture in this case denoting the alignment of the front sight and target only). It was found that as long as the front sight was viewable through the aperture, the POI followed the front sight, without regard to the location of the front sight within the aperture.

The short story for why this happens is that it is the result of the aperture being smaller than the pupil of the eye. The sight aperture becomes an artificial pupil and defines the "center of perspective".

While the paper, as written, addresses military style aperture sights (use of a front sight post, as opposed to a globe front sight), the results are applicable to target sights. With a standard globe front sight I would expect the POI to be unchanged as long as the front sight globe is fully viewable within the aperture. The extent to which it is "centered" within the aperture will not effect the POI. While I do not advocate intentionally not aligning the front and rear sights, I believe this discovery indicates that there is no need to "obsess" over the alignment, either. Full attention can be paid to aligning the target and front sight, where it is needed.

Here is a link to our paper:

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

Two college doctorate holding individuals got a multi million dollar contract to examine whether or not the skill of the coach had anything to do with the level of skill displayed acheived by the military trainee, They determined that it made little difference in outcome, They also discovered that to flinch was natural. Oh well. Now to the issue. What do the authors know about shooting, How skilled were the shooter subjects conducting the experiment? My obvious point is that alligning sights is not a natuyral act but a complicated shooting skill'that some shooters never master. A large grain of sault might go well with this hypothisis. Good Shooting Bill Horton

Bill,

I was one of the authors of the paper, and I conducted the shooting test. I have been shooting NRA Highpower for the last five years and have a Master's classification.

I'm not surprised that you are skeptical. I offer the following simple experiment to demonstrate the effect I am talking about:

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.

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.

Morning All,
I shoot small-bore 50m and 300m standard rifle.
While i agree with the purely mechanical/mathematical findings concerning sight alignment, i think your missing the point.

One of the main reason's for centering the iris and ring of the front sight is that it is an invaluable aid to correct and consistent head/cheek placement . Inconsistent cheek pressure on the rifle stock ( especially with the lower projectile velocities of small bore and air rifle) has a far greater effect on shot placement than a fraction of degree of MOA sighting error.
When your target at 300m is 5 ft x 2 1/2 ft it's not such a big deal, a competition shooter however is attempting to hit a target with a 2 inch diameter (the x or Mouche) and to do that with any regularity the rifle must be mounted and held the same way each time for up to 75 shots.

Correct and consistent sight alignment is an invaluable aid to maintaining consistent position and as such should not be underestimated or forgotten .

Jonno.

Jonno,

I agree with your statement about needing a consistent head position. In our paper we mention that fact. It seems that you agree that "sight misalignment" does not directly alter POI. The secondary effect of changing the hold on the rifle and its recoil behavior is nearly impossible to quantify.

It would be a very interesting experiment to conduct with a smallbore target rifle. The amount of movement involved to misalign the sights as described is VERY small ( something less than 0.5mm with a typical aperture), and would not necessitate and change in cheek position, a slight tilit of the head is all it would take.

I totally agree that a significant change in head position will change the POI of the shot. My qestion to you is this: Do you think that the amount of movement I am describing is "significant"?

Robert,
Fasinating bit of research. I have several questions.

Was the rifle used sighted in before the experimental shots? A "mobile" sight picture might make sighting in a rifle very difficult. While it might not affect performance if the rifle were already sighted in for that target at that range.

Your mention of the shooter eye and the apeture being a "geometric" intersection, would cause us to look at the range of an error factor in a rear apeture. The angle of change allowed in a 1 mm rear apeture would not make a noticable change in the geometry of the sight image, thus no change in the POI. What about a 1.5 mm rear apeture? Would that introduce enough of an angle change to the "geometric" intersection, to change the POI. Your research would then point shooters to use a smaller apeture for more consistant results.

Your discussion on parrallax suppression and parrallax reduction was excellant, and supported the concept of a "target centered front sight" but as another poster mentioned, shooter position would be the cause of not aligning the sight picture. This change of position would be difficult to overcome even with the "mobility" of a rear sight alignment.

What about a general statement that the rear sight alignment checks the shooter position relative to the rifle, and the front sight aligns the rifle/shooter to the target.

Joe

isuguncoach wrote:Robert,

Was the rifle used sighted in before the experimental shots?

Your mention of the shooter eye and the apeture being a "geometric" intersection, would cause us to look at the range of an error factor in a rear apeture. ... Your research would then point shooters to use a smaller apeture for more consistant results.

What about a general statement that the rear sight alignment checks the shooter position relative to the rifle, and the front sight aligns the rifle/shooter to the target.

Joe

Joe,

Yes, the rifle was zeroed before testing. The first group pictured is and indication of this.

The idea of geometric intersection can be explained thus: imagine a "beam" of light from the position of the front sight exiting the rear sight aperture. This beam of light will produce a spot of light on the pupil of the eye. This spot will contain all of the visual information concerning the target and the front sight. As long as this "spot of light" falls wholly within the limits of the pupil, parallax is suppressed and POI will follow the front sight regardless of sight alignment.

A smaller aperture provides allows for more misalignment without a shift in POI because the spot of light on the pupil is smaller and can be farther off center before it reaches the edge of the pupil.

I think your final comment is very useful. Sight alignment as an indicator of position, instead of being considered part of the "aim".

As I asked above, do you think a fraction of a millimeter of head tilt would cause a measureable shift in POI? Certainly moving the head forward or backward on the stock (while maintining sight alignment) would cause problems, as would changes in cheek pressure. Assuming that the cheek position and pressure are the same, but the head is merely slightly "tilted", is this likely to be problem?

isuguncoach wrote:Robert,

Was the rifle used sighted in before the experimental shots?

Your mention of the shooter eye and the apeture being a "geometric" intersection, would cause us to look at the range of an error factor in a rear apeture. ... Your research would then point shooters to use a smaller apeture for more consistant results.

What about a general statement that the rear sight alignment checks the shooter position relative to the rifle, and the front sight aligns the rifle/shooter to the target.

Joe

Joe,

Yes, the rifle was zeroed before testing. The first group pictured is an indication of this.

The idea of geometric intersection can be explained thus: imagine a "beam" of light from the position of the front sight exiting the rear sight aperture. This beam of light will produce a spot of light on the pupil of the eye. This spot will contain all of the visual information concerning the target and the front sight. As long as this "spot of light" falls wholly within the limits of the pupil, parallax is suppressed and POI will follow the front sight regardless of sight alignment.

A smaller aperture allows for more misalignment without a shift in POI because the spot of light on the pupil is smaller and can be farther off center before it reaches the edge of the pupil.

I think your final comment is very useful. Sight alignment as an indicator of position, instead of being considered part of the "aim".

As I asked above, do you think a fraction of a millimeter of head tilt would cause a measureable shift in POI? Certainly moving the head forward or backward on the stock (while maintining sight alignment) would cause problems, as would changes in cheek pressure. Assuming that the cheek position and pressure are the same, but the head is merely slightly "tilted", is this likely to be problem?

Hi Robert,

I test .22lr amunition lots about 5 five times a year in my rifle. I'm sure you're familiar with the procedure,it normally involves clamping the rifle in a fixed rest and shooting for group size. This is normally done with the rifle in full configuration ie sights and all attatched. I can say from my own observations, that when the rifle is sighted and correctly centred on the target, I can percieve absolutely no change in relation of the front ring to the target simply by moving my eye around the diopter, so I have to agree that misalignment of the sights in itself has very little if any effect and is to all intents and purposes insignificent. (I read in the book "way of the rifle" that the average human eye is able to aim/see to within 2mm using a normal diopter sight, so I can well believe your 0.5mm figure)
However, it does take a conscious and noticeable amount of movement of the head to do this, and I believe the difference in forces applied to the rifle stock and the subsequent effect on the rifles recoil pattern do have a significant effect on the bullet impact point. This is much more apparent with smallbore and air rifle due to the slower velocities encountered than with full bore rifle.

In conclusion I have to say that (in my opinion) correct and consistent sight alignment to a rifle shooter is a very important thing. Inconsistency and the inability to easily find correct sight alignment is usually an indication that somthing else is wrong with the position, and as such the importance should not be ignored.

Please excuse me if you have difficulty with my post, english is not my first language.

Rgrds, Jonno.

Anonymous wrote:
In conclusion I have to say that (in my opinion) correct and consistent sight alignment to a rifle shooter is a very important thing. Inconsistency and the inability to easily find correct sight alignment is usually an indication that somthing else is wrong with the position, and as such the importance should not be ignored.

Please excuse me if you have difficulty with my post, english is not my first language.

Rgrds, Jonno.

Jonno,

No excuses needed, your writing is just fine. I am "uni-lingual" so you know my language infinitely better than I know yours!

I appreciate your confirmation concerning the sight alignment phenomenon, that is very useful information.

I think sight alignment as an indicator of a consisitent position is an important concept. I also think that reasonable care should be exercised to align the sights for this reason.

I do think that most people have a misconception concerning this issue, though, and think that a misaligned sight will lead directly to a shift in POI, and that the sensitivity to misalignment is on par with that when aligning the front sight and target.

I don't think that small amounts of sight misalignment will cause problems as readily as similar amounts of misalignment between the front sight and the target. Because of this, I am suggesting that more of our available attention be applied to aligning the front sight and target.

It could be argued that both should be done at the same time (sight alignment and sight picture). This is valid, but I also know that concentration takes its toll on the eyes (and the mind) and I think that an over-emphasis on "perfect" sight alignment may not be a good investment of time and effort.

Robert Burdge:
I am in agreement with your theory.
I am prone shooter 50 m with years of experience in ISSF competitions.
I remember competitions in which, in spite of not being centered the rear sights, the results were inner tens.
When the rear sights were not aligned they were a warning: my head is not perfectly placed in the stock but it did not prevent a perfect execution of the shoot.
A coach taught to me, years ago, to aim without diopter and, once perfectly relaxed, he placed the diopter (rear sight) in its location without I moved my position. The diopter had to be left trim with my eye perfectly.
Thanks to share its study.

Interesting topic. While I have not done any research on the topic, a couple of interesting thoughts occurred.

1. If you look at the rear sight (at the 12:00 position) and place it (the 12:00 position of the rear sight) just touching the 12:00 position on the front sight (Apperature) with the bull centered and shoot a 10. Now if you reverse the position and hold the 6:00 part of the rear sight at the 6:00 position on the front apperature with the bull centered than you will still get a 10?

2. If this is true, then what is the purpose of having an adjustment on the rear sight?

I try to center the rear sight with the front sight, them forget about it. I find if things are centered the sight picture is crisp and clear. When misalignment occurs things tend to get somewhat fuzzy, leading to sighting errors.

Clamping the rifle and moving the head around does nothing as the rifle is aligned and no matter what you see the rifle will (supposedly in a perfect world) put the shot through the same hole.

I shoot (practice) every day and will run some tests tomorrow to see what happens. I certainly agree that some mis-aligned shots are good 10's, but was the mis-alignment compensated by bull centering issues?
TM

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/# ... eSightDemo

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."

Edited to add:

After re-thinking this matter I realize that my rationale in this post is not correct. Sight alignment would only need to be small compared the desired size of the group, not the size of the sight corrections.

I don't claim to have ever shot High-Power but I have shot quite a bit of Smallbore and Air Rifle and I feel that the conclusion of this paper is incorrect (just wanted to get that out there).

Now, I feel that aligning the target and front sight is very important to POI and that it is more important than aligning the rear sight and front sight but the importance of the latter alignment cannot be underestimated. If this alignment does not affect POI then why would anyone "sight-in." I don't know enough about the sight adjustments on a rifle used for High Power to make this statement for this kind of shooting but for air rifle and smallbore, I would venture so far to say ALL sight adjustment abilities on a rifle are found in the rear sight. If the alignment did not matter these sight adjustments would not make any difference. I don't think that Mr. Burdge would allow me to change his rear sight all I want just before he shoots a match and I would never allow anyone else to do the same to one of my rifles.

Of course I realize that this argument has been addressed in the previous post but I'll share with you all my own epiphany. I don't see any problems with the conclusion of the second paragraph of that last post, the "swamped out" one and I agree that no man (or woman) could have their aligment that perfect each time but I do feel that the adjustments of sights have an effect because it moves the AVERAGE of your sight alignment mistakes. I don't think that anyone can align the front sight within the rear with the precision stated in that post but let's say that he/she can align within 1/10th of its width (as opposed to 1/60th). When you move the sight but continue to attempt to align the sights your sight alignment "group" (I say group because the sight alignment problems can be compared to the target-sight alignment) changes causing your average POI to change.

Will,

I would like to respond to your message in two parts:

First, I agree with the last part of your post concerning my statements about the sight alignment needing to be small compared to sight adjustments - I didn't think that through carefully enough before I posted it. I have edited my previous post to reflect this.

Second, you said:

Now, I feel that aligning the target and front sight is very important to POI and that it is more important than aligning the rear sight and front sight but the importance of the latter alignment cannot be underestimated. If this alignment does not affect POI then why would anyone "sight-in." I don't know enough about the sight adjustments on a rifle used for High Power to make this statement for this kind of shooting but for air rifle and smallbore, I would venture so far to say ALL sight adjustment abilities on a rifle are found in the rear sight. If the alignment did not matter these sight adjustments would not make any difference.

It is incorrect to equate sight alignment with eye movement, when dealing with small apertures. I do not think that you took the time to read our paper, or attempt the simple aiming exercise that I described in my second post, or you would understand this. I would encourage you to examine the evidence supporting my statements before forming a conclusion. If you do so, I think you will find that my statements concerning sight alignment are correct. Since publishing this paper we have received many replies from individuals indicating that they have duplicated our experimental results.