鉄砲道 — teppoudo

Stereopsis (derived from the greek word stereos meaning solidity, and opsis meaning vision) is the process in visual perception leading to perception of stereoscopic depth.

This is essentially the process where the brain takes the distinct images from each eye and combines them into an image that is more than the sum of the parts since it is a 3D image which allows the perception of depth.

There is much debate on shooting with one eye vs. shooting with both eyes open. The existence of stereo vision makes shooting with both eyes open for many people (including me) very difficult.

The proponents of shooting with both eyes open cite various benefits for doing so. Taking the “tactical” reasons out of the equation, there are still a number of reasons presented, some of which are summarized below.

Stereo vision is needed for depth perception, which becomes important when shooting moving targets, especially 360 swingers that need to be rotated. With only one eye open, it becomes hard to figure out whether the paddle is coming towards you or moving away.

Eye fatigue is often sighted as a reason with the claim being that vision deteriorate faster when only one eye is open.

Another reason raised relates to not going against our natural tendency. We tend to do everything with two eyes open, so trying to shoot with only one eye open would not be natural. Even though it’s not strictly about vision the quote below seems to make some sense in this context.

If by nature you do things quickly, or slowly, you’re going to swing the golf club basically the same way. forcing yourself to an opposite extreme is rarely going to work because its too contrary to your basic instincts or impulses– especially when your under competitive pressure.

— Jack Nicklaus

Given all the benefits cited, I thought it would be a good idea to at least attempt to teach myself to use both eyes while shooting. I wanted to do it the “natural” way, i.e., no tape on the glasses or turning my head to odd angles etc.

The main issue I ran into was related to stereo vision and how the brain combined the images from both eyes.

The framing test is a good illustration of the issue I ran into.

The image below (obtained from a thread the Glock Talk) shows the double images produced because of parallax.

Sometimes the two targets had a big overlap between them and appeared as a blob.

If there is a strong dominance in one eye, that eye will take over and produce a better defined image and the image from the non-dominant eye will appear very faint and can be easily ignored. You can confirm this by squinting (not completely closing) one of your eyes and trying the above test.

In my case, my dominance is not strong, hence I see an image very close to the one above.

Now I know that I need to shoot the target on the right since it’s the one being picked up with my right eye and I need to use the sights on the left since they are the ones being picked up with my right eye.

This was not an acceptable solution since if nothing else it is a big safety issue. I don’t want any ambiguity in what I’m shooting at. Another issue comes up in target arrays with multiple overlapping targets. Essentially it’s a mess.

In order for this to work at all for me, I needed to see only one target downrange, not a real target and a ghost target.

I didn’t really care that I saw two sets of sights, since my body index made it very easy for my brain to pick up the correct set of sights as the gun came into view. In order to teach myself that, I used the exercises described on pages 170 & 171 of Practical Shooting – Beyond Fundamentals by Brian Enos.

Now if I look for it, I can still see the image of the sights seen by my left eye appear to the right of the set that I’m using, but if I don’t explicitly look for them, they don’t distract me at all and aren’t there for all practical purposes.

When thinking about the ghost target problem I realized the following. When I focused on the target, I saw only one target. As I moved my focus back to the front sight, the target would get blurry as expected and the ghost target would separate from the real target and move to the left. The interesting thing was that there was a slight lag between the time I had the front sight in clear focus (with the target blurry) and the time when the ghost target appeared.

This time interval was extremely short, but it was there.

This indicated that the brain was making a decision at some point after I had my clear focus on the front sight to show me both images. Before the two images appeared, the single image was aligned with the sights and my right eye.

If I could stop the brain from proceeding with the creation of the ghost image, my problem would be solved. Incidentally the only reason I am calling it a ghost target is that it’s not the one I want to shoot at. It’s simply the image from my left eye and nothing more. It has no special significance

Since I already knew what I wanted to see, and I could visualize it, I consciously tried to slow down the creation of the second image of the target. It took a while, but it started to work. Eventually the second image disappeared completely.

Initially this process seemed to be linked to distance from the target, e.g., I could do it with a target at 10 yards, but a 20 yard target was problematic. After working on it for a bit, distance is no longer an issue. I suspect distance was initially a factor since I was attempting to use depth of field of solve the problem, instead of convincing my brain to image correctly.

Now I see two sets of sights (with one set really easy to ignore) and one target regardless of where I focus. If I switch my focus to the target, I have full depth perception.

When I switch focus back to the front sight, the target stays put. Finally

All in all this process took me 4 months with a ton of dry practice. I suspect I would be able to do it a lot faster now since I know what I need to do.

I have not noticed any impact on any other activities.

I’m not yet sure if there are any significant advantages to shooting with both eyes open, however I can confidently say that I’ve noticed no disadvantages for me in the context of IPSC competition and intend to continue shooting this way.

Incidentally eye dominance didn’t seem to matter. I am cross dominant and haven’t had any issues after training myself through visualization.

Update (April 22, 2007): Please refer to the post on eye alignment and eye focus for further an attempt at an explanation for how I managed to stop the brain from creating the ghost image of the target.

Update (April 23, 2007): I’ve added a post describing the training method I used to learn how to shoot with both eyes open.

Update (May 10, 2007): Added the image of the pistol sights above to clarify the post.