"For most people, shotgunning is easier and more effective with both eyes open. Binocular vision facilitates the estimation of distance speed and angle, and helps one to get the full benefits of hand-to-eye coordination (if you doubt it, try catching a ball with one eye shut). There are other benefits to binocular vision such as reduced tension and fatigue. However, do not believe those who tell you simplistically that everyone should shoot with both eyes open. It is just not that simple (which condemns more than a few shooting tomes to the pyre): the critical consideration is eye dominance.
It is a curiosity of the binocular system of human vision (at least in most adult males) that one eye tends to control pointing. If a finger were pointed at a distant object, there would be a straight line relationship from object to finger to eye. The pointing eye, whichever it may be, is called the dominant or master eye. The majority of adult men have eye dominance which matches their handedness and, once this is confirmed, are well advised to shoot with both eyes open. There are other possibilities. Some may be cross-dominant (e.g., right-handed with a left master); a few have central vision (neither eye dominating); and others may be predominantly but not fully dominant in one eye. For those shooting a standard gun who fall into one of these categories, the best advice is usually to shut or squint one eye. In women and children, absolute dominance in the eye overlooking the breech is the exception, and one-eyed shooting is often the simplest remedy too.
Eye dominance is an intriguing phenomenon in which biological, environmental and experiential factors appear to play a role. Eye dominance in boys typically becomes more absolute with advancing years. In middle-aged men, however, it may become less absolute. It may be affected by training (disciplining oneself to sustain focus on the bird and ignore any “ghost” image), but results are unpredictable. It is (largely) unrelated to visual acuity (one can have poor vision in one eye yet it can still be more dominant as far as the control of pointing is concerned). It can vary in the same individual. It can be disturbed by fatigue, ill-health, staring at computer screens, long-distance driving and low light levels. It is not just a physical phenomenon, but a mental one as well. Having considered some of the scientific literature while researching this book, it appears that gunfitters may have a more profound understanding of eye dominance than anyone else (especially with regard to sex and age differences).
The diagnosis of eye dominance certainly involves far more than a simple – and potentially inaccurate – observation that an individual is right or left eye dominant (any testing method that only gives “either/or” results is worthless). It is common, for example, to find a male client who has what might be called “pseudo-dominance”, i.e., when tested, one eye appears to be almost – but not quite fully – dominant. Such a condition is easily overlooked by an inexperienced or sloppy instructor (typically being misdiagnosed as full dominance). However, the effects on shooting can be profound. Typically, there will be many inexplicable misses on quartering and crossing targets where the lead does not favor the dominance.
It is not uncommon, moreover, for shooters to be wrongly advised to switch shoulders having been told they were cross-dominant, when in fact their dominance in the opposite eye was not absolute (much better and simpler advice would have been to stick to the “strong” shoulder and squint an eye). All of which leads me to conclude that the precise diagnosis of a client’s eye dominance is one of the most vital considerations in shooting instruction. One need make no apologies for dealing with it in the most scrupulous manner….
A right-hander with a left master eye (or a left-hander with a right master eye) has a number of options. One of the easiest, in the former case, is to shoot from the right shoulder but closing or dimming the left eye prior to firing. Rather than keeping the eye shut throughout the pickup, swing and mount, it will be better for most sporting and game shots to dim the eye as the gun comes up to the shoulder. This way one gets some of the benefits of binocular vision and has an increased field of view during the critical pickup phase. It is a definite mistake to dim the eye only at the last moment as this may be visually confusing.
The offending eye may be covered with a patch (although instructors who inflict this on novices should try it themselves), or if the student wears spectacles, a block to vision may be placed over the appropriate lens. This need not be a full-sized patch but may be a much smaller block, refined so that it is no more than a half inch across. One may use electrician’s tape, a smudge of Vaseline, chapstick, typing correction fluid or a Magic Dot on the lens to achieve this (once the position and size are confirmed, glasses may be permanently and neatly modified by sandblasting in an optical workshop)….
Kay Ohye, the famous trap shot, developed a “blinder” to be attached to the rib near the muzzles of over-and-unders. It has subsequently been manufactured by several firms. Another clever device consists of a U-shaped channel with a fluorescent sight at one end. This may be attached to the barrel and when in place, the brightly-coloured insert can only be seen by the eye looking along the rib. A similar effect may be achieved by using the thumb on the forend to block the vision of the eye not looking down the rib (as practiced by both Churchill and Barry Simpson)….
A traditional remedy for those whose master eye and handedness do not correspond, is to learn to shoot from the left (or weak) shoulder with or without a suitably adapted (cast-off changed to cast-on triggers reshaped) gun. I do not usually favour this course (though it is sometimes appropriate). The advantage of binocular vision may be outweighed by the awkwardness of the manoeuvre. It is my experience that few of those who are forced to take this route develop into really first class shots (although many one-eyed shots, beating the odds, do)….
Finally beware: not everyone who shoots as their eye dominance – as tested – might suggest. Some may be able to shoot well with both eyes open, even though initial testing without a gun indicates a dominance problem. Some will have inconsistent eye dominance (my own normally right-eyed dominance fades and can even switch when I get tired). Some will have a master eye significantly weaker than the other as far as visual acuity is concerned. Inability to focus can undermine eye dominance. (Sometimes those with contact lenses who have a slight eye dominance problem, may be advised to shoot with the left lens removed.) Some people’s eye dominance is affected by the choice of gun. Short guns and side-by-sides tend to cause more problems than the more pointable over-and-unders. Changing to longer barrels can help someone with a mild eye-dominance problem…."
The Shotgun, a Shooting Instructor’s Handbook, Michael Yardley, copyright 2001, Safari Press
The problem with cross-eye dominance is that your brain naturally "ignores" a lot of the information from the "weak" eye. It's basically there to gauge distance - parallax - not show detail. Detail is what you want in a precision sight picture. 
