centering eye on rear aperture

[jcerne]

Target shooters are often told when using aperture sights that they need to center their eye behind the rear aperture so that the front sight is also centered in the rear aperture. I’ve seen some very high level coaches state this, but I don’t think it’s correct and I just did a simple test to convince myself that this is the case. I would claim that if the front sight is aligned with the target bull and one is looking through the rear aperture, the rifle is well-aligned to the target even if the front sight is not perfectly centered in the rear aperture. Of course, this is not true for open/pistol sights, where the target, front sight, and rear sight must be aligned to the shooter's eye. In this case the shooter’s eye and rear sight are far apart and the front sight must be centered on the rear sight. With a peep sight, the rear aperture and eye are in nearly the same place. Of course, one should try to center the front sight on the rear peep to maintain consistent head position and to get more light into the eye, but I don't think that this centering is as critical as for aiming with open sights. Part of the reason is that the rear aperture is very small; it only looks big because your eye is right up against it! So moving your eye behind the aperture and looking at light rays going through different parts of the aperture, doesn’t change the direction of the rays very much at all. Please see below an excellent article on parallax suppression:

https://www.google.de/url?sa=t&rct=j&q= ... UrveUl3l9P

This is a bit counter-intuitive, but a simple experiment helped to convince me. If one carefully aligns the rifle on a solid rest onto the target and then moves one's eye around behind the aperture WITHOUT MOVING THE RIFLE, the front sight will still remain aligned with the target bull (even if the front sight is no longer centered on the rear aperture) and the rifle will still hit a 10. I just posted a video on Youtube demonstrating this:

https://youtu.be/SXQRff4p2vw

Please note that the position of the target bull on the front sight aperture doesn’t change as the camera moves around behind the rear aperture, causing the front aperture to no longer be centered on the rear aperture. The bull is a bit to the left in the front aperture, and stays that way no matter how I move the camera.

I hope that this isn't considered to be blasphemy or even worse, totally obvious! As someone who's interested in optics, I find this result pretty interesting.

[Pat McCoy]

You are right. Attached are a couple of PDFs from a few years back, which have a similar explanation.

[spektr]

You are correct qith what you are saying, but there is another phenomenon at play besides alignment. One reason Irons are such a great sighting aystem is that you are constructing a rudimentary telescope. When the pupil is placed in the centered position, your eyeball being a component of the telescope, the image of the target will enlarge, andd the effect looka like it jumps closer to the eye.

I haven't heard you describe this yet

[Tim S]

I remember similar threads before. The study the OP referenced, has been brought up before too. I agree that a small, and very consistent misalignment within the rear aperture can make no difference. But no-one has properly explained how the rifle will stay perfectly on aim with the eye wandering around behind the rearsight, as if the cheekpiece was not supporting the head. The study the OP referenced had the rifle clamped, so of course the alignment to the target won't change. This forum isn't intended for clamped rifles though.

[Moy]

Right, it's a difference of about 0.2 scoring rings on air rifle, if you get it wrong by the maximum possible amount. Doesn't matter for most of us. I suppose some people are at a level where 0.2 matters.

Consistent head placement matters more if you are shooting scoped rifle with no or improper parallax adjustment, and are shooting extremely close. For an extreme example, if I practice at say 8 yards with a 10 yard parallax setting, and a 24mm objective lens, I can make shots be off by two full smallbore scoring rings by messing up head position. In practice though, for high power, you are shooting at 200-600 yards, with 200y+ parallax, and your maximum possible error is less than 0.1 scoring rings (and there's no fractional points ever, so it literally doesn't matter).

[jhmartin]

.....
I suppose some people are at a level where 0.2 matters.

i.e. anyone shooting International Air or Smallbore!!!

[rgibson]

Misalignment of the front and rear sight causes the angle of dispersion to increase as range increases whether shooting at 50 feet or 1000 yards. But I only shoot from the sling at 50 feet to 1000 yards.

[Moy]

.....
I suppose some people are at a level where 0.2 matters.

i.e. anyone shooting International Air or Smallbore!!!

Yup, exactly.

Here's a video from MEC on it:

https://www.youtube.com/watch?v=UI0kFTDPr8w

[jcerne]

Thanks for your informative and helpful replies! I learned a lot from them. The original video from MEC that got me thinking about this was at:

https://www.youtube.com/watch?v=ku2-izm-N4A

This video shows the POI changing by several rings due to parallax while the bull remains centered on the front aperture sight, which I don't think is correct. I wrote to Heinz Reinkemeier about this in March 2018, and he actually replied! It's nice to see that he made a new video, which is more accurate. Thanks Moy for letting me know about the new video.

[Bob Foth]

If you think about keeping the eye, front sight and target aligned, but move the rear sight/rifle around, I think it is easier to see the reason misaligned sights cause errant shots. Most ISSF event shooters use rear iris settings in the 1.0 – 1.3mm range, depending on lighting, etc. Even at the smallest setting of 1.0mm the maximum error in one direction would be a 0.5mm radius. For 10m air rifle, the distance to the target is roughly 12x the sight radius. Using similar triangles that would mean an error of 6mm on the target. Each tenth of a ring is 0.25mm so that shot would score 8.5. Realistically, even being off by ¼ of the rear iris radius would give you a 10.3 in a game where you need to average about 10.5 to make an ISSF Final.

Other factors to consider include:
Changing your head position can change recoil, resulting in shot displacement (particularly in smallbore). It is also a sign of inconsistency in your position overall.
A well-matched lot of ammo still adds another tenth or two to your “error budget”.
The 50m target rings are about 36% smaller proportionately vs. the 10m rings, exacerbating the problem in addition to rifle/ammo combinations that are usually less accurate. 300m is slightly more forgiving, but still smaller than 10m.

The Burdge&Kerr study uses illustrations and examples that are so far removed from the precision required for Olympic style shooting that they don’t seem relevant to the scale we are working with on every shot (scored to 0.25mm on a target 10m away). Even if the rear iris is smaller than the pupil of the eye and there is some “parallax reduction”, unless parallax is reduced to zero, it will impact the score.

While the MEC (Duplex sales) video referenced by Moy has beautiful graphics and agrees with my perspective on sight alignment in the script, it actually has the movement in the image of the shot displacement (red ring) reversed from what would actually happen (sorry Heinz!). It looks like the video jcerne refers to has the directions correct.

[groverdog1]

Then the 50 year practice of "shading" shots in the wind using offset sight alignment has been a farce? I don't think so........

[Tim S]

Then the 50 year practice of "shading" shots in the wind using offset sight alignment has been a farce? I don't think so........

But that's normally by displacing the target in the foresight, not misaligning the foresight in the rearsight...

[rgibson]

Intentionally misaligning front and rear horizontally to compensate for wind....I think you are proving our point.

[rgibson]

If you are are talking about favoring, the front and rear should still be aligned. You would be aiming off to one side of the target or the other depending on the wind. Misalignment instead of just holding off the amount you value the condition sounds too much like voodoo.

[Tim S]

Intentionally misaligning front and rear horizontally to compensate for wind....I think you are proving our point.

Quite. Shading by aiming off centre is hard enough already. Why add extra variability by not using the sights properly?