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; functioning more like a nonmagnified red-dot sight, than a magnified scope.  However, 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 of sand bags, 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 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!

Yep, I had the same results with a TA33. Good job documenting it.

Yep, saw a lot of that with the original Single Point & Armson OEG way back when. Try your experiment again except don't spend a lot of time on each shot, do individual quick snapshots instead. The effect is reduced if you don't give the brain a lot of time to compensate for the differing inputs. You should see more centered groups.
Keep us posted!...
Tomac

Do you think the amount of deviation in point of impact would be any different with other ACOGs?  What about low power variable magnification scopes?

Why wasn't a similar size grouping attained while shooting the ACOG occluded?

That's  how I conducted the test.  If I took longer than a couple of seconds in aiming, I could literally watch the reticle drift farther to the right.

It's interesting to note that Trijicon's literature does indeed state that phoria is "time dependent."

Very nice, Molon.  Thanks again for yet another quality thread.

Very educational.  

Good post.  Remember, using the ACOG as an occluded sight would really only be useful within 10 yards or so.  25 yards is pretty extreme and there would be no advantage over the magnified optic.  CQB distances is where the occluded sight may be better.  It's not designed to be used as an occluded sight, this is just a variation of use.

My thoughts exactly.  Interesting thread though.

Excellent report. Thanks for posting.

Just a thought but at that range (25 yards or less) would the groups be closer to point of aim if you just looked over the top of the scope?

I have never used an ACOG.....just fingered a few

I'll be putting that theory to the test in the near future.

Hope I dont miss that thread.


I think I know how it'll turn out

It is also dependant on head position and eye distance.  If your head is square your off eye is further to the side inducing more shift than a head that has one eye more forward and thus closer to the centerline of the scope.  Also if your head is level you get pure horizontal shift vs the elevation change you noted which looks like your head is slightly canted.

Phoria is a lazy eye or drifting eye and not the same thing as the shift you get from using an OEG and not having the same sight picture in both eyes.  You are discussing two things in this thread and I think most people will think it is one.  There is a difference of view shift (difference due to position of non dominant eye) and a phoria shift (wandering of dominant eye) which are not the same thing.

There are two things that can cause the increase in group size... one is that you may keep your dominant eye in the centerline of the scope but if your off eye moves it causes the POA/POI to shift... so if your head gets more level or you move your head rotationally slightly you get reticle shift. The other is phoria or a wandering of the dominat eye and casues its own shifting.

A 1X optic allows for the dominant eye to view straight down the tube and get a binocular fusion of the FOVs from both eyes.  This is why people want true 1X optics... you dont get a shift as seen when using an ACOG as an OEG or in BAC mode before trasitioning to the dominant eye's view and making the correction.  I have found similar shifts in both TA11 and TA31 ACOGs.  THe issue is not magnification but the non dominant eye being off center... that part does not change with magnification.

I had an Armson OEG back in the 80's and also experienced this. I also tried to make quick 'snap' shots.

The effect will be different for each person, but, they will experience it.

It is, at best, a very close range CQB engagement method.

Good post/write-up.

Thanks for the good post. I've always swore my ability to use my TA33R-8 OEG style was equal to any red dot sight I've used before but that was based on my ability to quickly and immediately attain my target when looking throught the scope without actually ever having tried shooting it that way. Looks like I'll need to actually shoot it this way before I make any more claims!

Interesting information.  Although, I do wonder (eventhough it's an option) why you would want to use the ACOG as an occluded gun sight over BAC?

Becasue the BDC is calibrated for 100-800m and using it as an OEG is for  close range 1X capability at short range.  I dont see whats confusing about that. If you meant BAC instead of BDC... BAC is just using the the optic as an OEG in order to get on the reticle on target before allowing your eye to go to the zoomed view to make the final adjustment in magnified mode.  A magnifed optic at close range is slower and harder to use than a 1X optic and this offset of the POA/POI still happens in BAC mode with the quasi 1X view as the view of your off eye is still offset from the centerline of your scopes view.  If you force your brain to stay in a 1X view you still lack binocular fusion and can get the phoria effect the longer you stay in that mode.  By simply occluding the front of the scope it becomes easy to stay in 1X view while the scope is not swinging and you also dont get reticle dimming when using a flashlight in the dark which makes staying in 1X mode during BAC easier since a dark reticle makes seeing the 1X view more difficult as well.

I agree that there are really two separate phenomena being demonstrated here.  The first is the group size increase due to phoria, and the second is POI shift due to eye dominance.

I have never tried using my ACOG as an OEG, but I do routinely fire tritium-sighted handguns strong side with an NV monocular in front of my non-dom eye in very dark environments using the superimposition technique.  There is a significant POI shift when one eye exclusively sees the sights, and the other eye exclusively sees the target.  The magnitude of shift can be surprisingly high, on the order of 4-6" at 10-15y in my particular case.  The technique requires alot of actual practice for one to learn the shift characteristics as it is heavily range dependent even very up close and personal.

Great info.  Yes, I did mean BAC.  This info really shows that one optic cannot do it all.  I'm sure that after a learning curve that Phoria can be controlled to a point.  Although there is a significant difference in the two groups fired by the OP, it seems still within the realm of effectiveness. That being said, if I had the extra $ I'd get an ACOG w/ a docter on er' or a seperate Aimpoint Mirco on a cant mount.  

Presents a tough choice for $ short guys like me.  I want to get an ACOG (I like the TA33 best) for the added distance it gives, but don't want to be caught flat-footed by something up-close.  Well, you just can't have your ACOG and eat it too....or something like that.