You can learn to use even high-power target scopes with both eyes open. The 20X Redfield on my smallbore prone rifle presents my dominant eye with an image starkly different than what my left eye perceives. But practice has enabled me to shoot with only a slight narrowing of my left eye--and that mainly to reduce sunny-day glare spared my dominant eye by the scope's eyepiece.

Open sights can be more difficult, as you're lining up three images in three focal planes. A double image of the front sight can delay a shot or cause a miss. An aperture rear sight absolves you of rear-sight focus, so shooting with both eyes is easier. Concentrate on your point of aim and center it with the bead. An aperture rear sight lets you aim a rifle much like you'd point a shotgun.

Optically a scope has an edge over iron sights because the reticle is in the target plane and will be seen in sharp focus. Your eye cannot bring a front sight and a distant target simultaneously into sharp focus.

Binocular vision is a marvelous gift. No matter how far you must look or how precisely you must aim, two eyes are truly better than one.

Two-Eyed Glassing
Some of these same principles apply to glassing for game. When I hunted Coues deer with David Miller a decade ago, he carried 30x80 binoculars, along with a husky tripod. He hauled both into the desert hills without complaint because after he sat down to glass, he'd peer through those lenses for hours on end.

For backcountry hunters who must stay mobile, spotting scope magnification is available in some binoculars. Zeiss and Nikon have built 20X and 16X glasses with stabilizers to reduce the image shake that afflicts powerful binoculars when supported only by hand. Leica's Duovid 10+15x50 binocular delivers a choice of 10X and 15X magnification. Doublers, like the detachable eyepieces by Brunton and Swarovski boost power without adding bulk or weight until you use them.