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

[Failure2Stop]

I noticed this a few years ago when we started to use the ACOGs intensively. I didn't know the name (I just referred to it as "crossed-eyedness"), but I definately knew the result.

I have never recommended the use of the ACOG as an OEG, mostly because of this distinct shift we see in about 50% of shooters to varying degrees and distances of effect. I use the OEG technique only as a method to get the shooters into shooting with both eyes open.

[rob_s]

I have experienced this as well when using a 3x and 1.5x Compact ACOG in an EAG class. Pat typically does a block of instruction where you close your front scope cap or tape up the front of the optic if you don't have a cap. I've seen a shift in my groups, even at the close ranges involved, every time.

[JSantoro]

The phoria effect is the reason we'll do occluded-view drills with the RCO strictly as a proof-of-concept to our students, too. They may be skeptical about shooting anything with both eyes open as a direct or indirect result of the Old Guard and their "iron sights were good enough for me" nonsense, or whatever other reasons. Usually done at distances no greater than 25m, and we see all sorts of results that don't appear to follow any one particualr trend, probably due to the endless variety of differences from one dominant eye to the next, plus the variables of the individual relationships to the off-eye.

They get hits on a Echo target, but that's about as much as you can say. From a confidence standpoint, we look upon it as enough to know that you can use the thing at close ranges even if it's busted. It's a simple tool, but the confidence boost a student may get from seeing scattergun hits from an occluded optic zip right to center mass once the tape is removed is generally helpful to us

I'm usually pretty jazzed when I see those few that manage to figure it out while the tape is still on, and can keep their prints somewhere within +/- 3" of the vertical centerline. Don't care if they're in between the preferred head and chest scoring zones, I just like to see the light bulb in their head flare up when they start to think about what the optic is doing for them.

[GreyOps]

I noticed this a few years ago when we started to use the ACOGs intensively. I didn't know the name (I just referred to it as "crossed-eyedness"), but I definately knew the result.

I have never recommended the use of the ACOG as an OEG, mostly because of this distinct shift we see in about 50% of shooters to varying degrees and distances of effect. I use the OEG technique only as a method to get the shooters into shooting with both eyes open.

To back this up, we would cover the front lens with tape. Again, to familiarize shooters with the bindon aiming concept and force them to shoot with both eyes open. I have used a covered front lens to reduce the time to acquire a target but only at 10yds and in.

[TomMcC]

Molon

This is what happen to me with my TA-11F. I thought it was age related. I couldn't use the scope anymore up close so I sold it. Use low power variables now.

By the way, I had the Progressive lense cover also. I tried the scope with and without it. Either way I got the huge POI shift you describe.

[RadioActivity]

Well hell, I learned something today!

I never knew this to be an issue, which is funny because I've shot with the bikini cover over my front lens on my aimpoint with no ill effect, but it was dynamic shooting now that I think about it.

Something to test on my own next time we have a range day. Thanks for posting this Molon.

[dfsutton]

I think this post has enlightened me to something that I have known about and have always had and could not put a name to.

While, I have never seen an opthamologist regarding this, I believe myself to have a very bad case of it. Because of this, shooting with two eyes open is difficult and shooting with an occluded front cap on a trijicon reflex is impossible for me. At 10 feet I noticed a POA shift of feet on the target. Not good.

[Muddyboots]

Every ones eyes ARE different! I shoot my Aimpoint M4S with the front cover on with only a slight loosening of my group. I know that I have strong yet damaged eyes. My normal 2moa dot looks like a little red spider about 4moa wide perched on the lens. It isn't the optic, it's my right (dominant) eye. My left is different and it looks like my little spider is on rappel. When I pop up my Troy rear buis and flip to the small app. my little spider obligingly pulls his legs in and I have a round 2moa dot. These effects stay the same in OEG mode as well. I'm fairly certain that eye exercises can make a difference in how this works. If you have specific structural issues it might be different. If you wear corrective lenses, your "lens train" is longer which might change things. Lastly, I wonder if the magnification has any effect on this. I'm guessing it does. Your lens cap is 4X (or whatever) closer even if it is a void black and the reticle is still over that void. (I know I'm not saying it well.) Anyway, we've discussed the front lens cover as a training tool and we've reenforced the fact that you have to test theory and see how it applies to you and your equipment.

It's all learning and that's cool!

Muddyboots

[uwe1]

You can test if you have phoria very easily at home. Take a small paster or target (anything to give you a very small point to focus on, like the letter "V" and look at the bottom of it) and tape it to the wall. Try to stand about 20 feet away from it.

Next, cover your left eye with your hand (playing card, 3x5 card, whatever works) and fixate on the point with your right eye. Then quickly move the occluder/your hand to the right eye and attempt to focus on the same point with your left eye. If you notice that the dot you were fixating on moves at all, then you have a phoria. You can then repeat this by moving the occluder from the right eye back to the left.

EDIT: It's important to cover the eye for at least a one or two second interval before moving it to the other eye. This will allow for a greater amount of dissociation so you can see just how big the deviation really is.

If the fixation dot/target moves "WITH" (in the same direction as) your hand/occluder moving, then you have a exo-phoria...your eyes naturally point away from each other or diverge.

If the fixation dot moves "AGAINST" (opposite direction as) your hand/occluder is moving, then you have a eso-phoria...your eyes naturally point towards each other, or converge.

If you notice that it moves up and down, then you have a vertical phoria.


The following is from another thread, but I thought it might make more sense to put the information into this one: (http://m4carbine.net/showthread.php?t=72211&page=2)

For example if you have a large Exo-phoria, where the resting position of the eyes point away from each other, your left eye would be fixated at a point to the left of where your right is fixating. The superimposed red dot would synch up with the left eye's fixation, even though the rifle is still pointed where the right eye is aiming. Usually, you have to be shooting at long distances for this to really matter.

Phorias are measured in prism diopters. To put it in a understandable unit of conversion:

Prism = 100 * tan (1 deg)
Prism = 1.75 prism diopters

While 8 degrees of deviation is:

Prism = 100 * tan (8 deg)
Prism = 14.05 prism diopters

So, someone who has a distance phoria of 14 prism diopters could experience a 8 degree shift on their POA/POI. They could also experience less than the full value of their phoria. It doesn't mean it will happen, but it could happen if they dissociate. Besides, exophorias at distances are more commonly around 1-6 prism diopters.

Keep in mind, one degree equals 60 minutes of arc.

Definition of Phoria: http://en.wikipedia.org/wiki/Phoria

The effects of phoria are present when one eye is occluded. The occluded eye has no signal to maintain fusion and thus will wander, generally to its resting position. However, people can consciously or subconsciously control their eyes to converge and diverge so when using the RDS/ACOG as an occluded optic, the zero will appear to wander, but will trend towards where the person's resting phoria will be. For quicker snap shots like in CQB type engagements, there is minimal shift as the eyes haven't completely dissociated yet. For longer range precision shots (anything that will require you to look through the occluded optic for one or two seconds), the POI will move in a generally predictable direction, but at a possibly inconsistent amount.

When you have a smaller phoria, the eyes are not very far apart at their resting position. For example, if you have a 1-2 prism diopter phoria at distance, your eyes will differ in angle by about one degree. If you have a 14 prism diopter phoria, the full resting value of the deviation will be 8 degrees, but the occluded eye will take time to get to the full 8 degrees (it has to wander out and the muscles have to relax). As the eye is moving out to the full value, the eye will have different degrees of deviation. Inherent muscle tone and voluntary/involuntary movement can also cause a fluctuation by a few prism diopters. This is why when you are testing for phoria (see above), you have to leave one eye occluded for a few seconds. The point is to completely break down that person's fusion.

I experience what Molon has described as well. I have a fairly consistent 2-4 prism diopter exo-phoria at distance. I don't experience any double vision because with both eyes open, my eyes fuse and compensate for the phoria. If you are unable to fuse the eyes, this is called a "tropia".

[uwe1]

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?

Not Molon, but this phenomenon varies depending on the type and magnitude of each shooter's phoria.