The Effects of Phoria When Using the ACOG as an Occluded-Eye Gunsight

[Molon]

The Effects of Phoria When Using the ACOG as an Occluded-Eye Gunsight


Phoria: the visual effect that occurs when one eye is blocked from seeing the same view of a target as the other eye; the blocked eye does not get the same sensory input as the other eye and can begin to wonder off, usually to one side or the other.

In an attempt to make my Trijicon TA-11 ACOG more versatile at engaging close range targets, I installed a Progressive Machine and Tool flip-down front lens cover. With the lens cover in the up position (blocking the view through the scope) the ACOG can function as a nonmagnified occluded-eye gunsight. When the ACOG is functioning as an occluded-eye gunsight, the view of the target for the eye looking into the scope is blocked and the effect of phoria comes into play.













In order to determine just what the effects of phoria would be when using the ACOG TA-11 as an occluded-eye gunsight, I conducted a test comparing the accuracy and points of impact when shooting with the lens cover down (normal sighting) and the lens cover up (occluded-eye sighting.) Testing was conducted at a distance of 25 yards, the farthest distance that I would anticipate using the occluded eye-method of sighting.

Shooting off a sand bag, I zeroed the TA-11 for point of aim equals point of impact at 25 yards using the normal sighting method. I then fired a quick 10-shot group with the normal sighting method that formed a tidy group with an extreme spread of 0.56”.




Next, I placed the lens cover in the up position, transforming the ACOG into an occluded-eye gunsight. The left eye views the target, while the right eye views the reticle in the scope. The right eye is blocked (occluded) from seeing the target due to the lens cover. The impact of the initial shots using this method had such a large horizontal deviation to the left that they were off the targets that I originally was using. I had to change to a 24” wide target and aim at the far right side of the target to capture the impact of the rounds. The aiming point for this portion of the test was the numeral “7” on the target.

Using the occluded-eye method of sighting, I fired a 10-shot group. This group had an extreme spread of over 7” and the center of the group was 14” to the left of the aiming point!






....

[BAC]

Wow. So leaving your lens cap on is bad, I take it.


-B

[ZDL]

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[SpankMonkey]

Good post.

Have you tried it at a shorter range? I would like to see what happens at 10 or 15 yards.

[RogerinTPA]

I didn't know the POI/POA would be that extreme. I was expecting a much closer group at your zero distance, within 3 or 4 inch spread (like the distance between your pupils). Much like shooting strong or support side urban prone, adjusting to the opposite side the RDS sight is on (3 vs 9 oclock). That is surprising.

[militarymoron]

that's interesting, molon. i'll have to try that out next time i'm at the range - maybe with an aimpoint (closing the front lens cap) to make it an OEG.
i'm wondering it the effect is consistent for the same shooter, or whether it varies, and how it varies for different shooters. when using the old armson OEG - which is an occluded gunsight only (not a view-through), would this phenomenon crop up, or does it only manifest itself when switching back and forth on the same optic, between looking through and using it as an OEG?

[RogerinTPA]

that's interesting, molon. i'll have to try that out next time i'm at the range - maybe with an aimpoint (closing the front lens cap) to make it an OEG.
i'm wondering it the effect is consistent for the same shooter, or whether it varies, and how it varies for different shooters. when using the old armson OEG - which is an occluded gunsight only (not a view-through), would this phenomenon crop up, or does it only manifest itself when switching back and forth on the same optic, between looking through and using it as an OEG?

Good point. I'll try it today when I go to the range. If it is consistent with the shot group Molan fired, you may have to zero with the front site covered, like the Armson OEG, to get acceptable accuracy, rendering the technique impractical to switch back and forth with the flip of a lens cap.

[militarymoron]

well, i gave this a try at the range today, and was pretty surprised at the results. i was shooting an aimpoint at 50yds. sighting through the aimpoint normally, with the rifle in a bipod and rear rest, the dot was centered on the target. i then flipped the front lens cap closed without moving the rifle (i did it a couple of times to make sure that it was on target).
i'm a lefty, by the way.
i then kept both eyes open, allowing the dot seen by my left eye to be superimposed on the target image seen by my right eye, which is how an OEG works. holy crap. the dot was about a foot high and two feet over to the right of the target. off the paper completely (POA high and right of POI). to correct, i'd have moved the dot down and to the left, which would have put the POI down a foot and to the left two feet - completely missing the target. i checked this out a couple of times.

i would have thought that using my left eye as the occluded one would result in the opposite effect that molon observed. it didn't. how curious. my POA is to the right of the POI, just like his. very interesting. what this says to me is that when using an ACOG, aimpoint or any other scope normally used by sighting through it, flipping the front cap closed to use it as an OEG will result in a helluva offset and render it practically useless for anything but up close and personal.
you could probably quantify that offset at given distances and hold for it, but under stress, will you remember? i'm pretty sure i won't. it's one thing to remember the height above bore offset at close range, but when it's a lateral+height offset, that gets a bit iffy IMHO.

molon - thanks for bringing this to our attention. i've used the 'you can also use it as an OEG' line before (when the battery is low on an aimpoint and it's too faint), but never quantified the offset (i thought it'd be minimal).

[FromMyColdDeadHand]

Does it matter how your handed-ness and eyed-ness coorelate?

[subzero]

molon - thanks for bringing this to our attention. i've used the 'you can also use it as an OEG' line before (when the battery is low on an aimpoint and it's too faint), but never quantified the offset (i thought it'd be minimal).

I talked to Ken Hackathorn about this effect a couple of years ago at a low light class. I tried using my Aimpoint in the manner you described (at his behest) and observed a noticable shift even at close range. 4-5 inches at least, at roughly 10-15 yards.

I asked him about it, and he said the amount of shift varies from shooter to shooter, presumably based on the shape of your eyes. Literally, the shape of your eyes. I would presume then it's one of those things like blurry Aimpoint dots, where the flatness of your cornea or astigmatism plays a major role in your perception of where or how you see something. My eye doc says I'm a bit cross-eyed, and I'm sure that has an effect in this particular application as well.

I still think the OEG is a valid method of persuing the conflict, but I know *I* have to aim right, maybe even way right.