Aiming with Both Eyes Open
Some of the best shooters in the world only use one eye when they aim. However, instead of closing their non-aiming eye---which is always considered gauche, due to sympathetic dilation of their pupils---they gently press a piece of Scotch tape over their shooting glasses such that the view out of the non-aiming eye is "frosted," i.e., light comes in, but there is no clearly visible image. For example, David Tubb, the world's greatest high-power rifle champion, uses Scotch tape to block the view of his left eye, and you can actually see this on the cover of his (outstanding) book, Highpower Rifle.
From what I can tell from watching recurve archery (both in person and on TV), all of the great archers shoot with both eyes open. Archers can "easily" use both eyes because their aiming process is simple compared to what the rifle shooter must accomplish. This short note provides experiments that will enable you to understand, first-hand, what is going on with regard to the "double" images that you should see while aiming with both eyes open, and will teach you which images to ignore! (If you cannot see multiple images when doing the following exercises, you might want to mention this fact to your eye doctor upon your next regular checkup.)
Understanding the Rear Sight
There are two kinds of rear sights, "near" and "far" (in my terminology). A "near" rear sight can only be seen by one eye, and this includes recurve (Olympic) bows (the drawn string is used as a sight), compound bows (peep sight), air rifles (rear aperture sight), and so forth. The only "far" rear sight that I can think of belongs to a pistol, and I will discuss them the next revision of this document. So, in summary, the (near) rear sight picture does not change when you open both eyes, because the sight is only visible to one eye, given its proximity to your face.
Understanding the Front Sight
If you focus on the front sight, you will see ONE front sight, and TWO targets. If you focus on the target, you will see ONE target, and TWO front sights. We should start by exploring this experimentally. Take a pen to be your front sight, and hold it out at arm's length, putting it just below a distant target. (Consider something like a small piece of fruit at something like 30ft.)
Let's start by staring INTENTLY at the target (pick out a tiny detail of it). You will notice that your front sight appears TWICE. Don't try to focus on the front sight, as the two images will "collapse" and fall together; instead, continue stare intently at the target, but observe the two front sights. Note that as you stare intently at the target, you can move "either" of the two front sight images under the target as your aiming index. Now, note that the LEFT front sight image is connected to your right eye (close your left eye and see that you're still aiming in the correct place.) Conversely, note that your RIGHT front sight image is connected to your left eye (close your right eye and see that you're still aiming in the correct place).
Aiming When Looking at the Target
The archer always keeps both eyes open and looks directly at a point on the target. If your "near" rear sight is over your right eye, and you're looking at the target, aim with the LEFT front sight image. If your "near" rear sight is over your left eye, and you're looking at the target, aim with the RIGHT front sight index. While a rifle and a compound bow have very clearly defined "near" sights, the string on a recurve bow (just in front of your eye) also behaves like a "near" sight (that has to be aligned with the riser), and this determines the front sight image that you must use. (Just one extra point on the recurve: As you're looking at the target, you will see two riser images, and will use the riser that is associated with the front sight that you use.)
Aiming when Looking at the Front Sight
When you look at the front sight, you will see two targets. Observe this yourself by looking at the tip of a pen, held out at arm's length. Be very careful to look at a detail on the pen, like a splotch of ink around the ball, or something exquisitely small. The target will immediately split into two. If your eyes dart back to the two target images, they will "collapse" and your pen (the front sight) will start to split into two images.
Go back to looking at your pen, and note that you can move it under the left or right target. By closing one eye at a time, you will realize that the LEFT eye is lined up with the left target, and the RIGHT eye is lined up with the right target. So, if you're shooting with the "near" sight over your right eye, and looking at the front sight, aim at the right target. If you're shooting with the "near" sight over your left eye, and looking at the front sight, aim at the left target.
CantingIf you cant your head (turn it side-to-side), and look at the target, for example, note that the front sight images track appropriately! it's best not to cant your head, if you can avoid it.
If you are RIGHT handed, the "near" rear sight will be in front of your RIGHT eye, and when looking at the target you aim with the LEFT front sight image, and when looking at the front sight, you aim at the RIGHT target image.
If you are LEFT handed, the "near" rear sight will be in front of your LEFT eye, and when looking at the target you aim with the RIGHT front sight image, and when looking at the front sight you aim at the LEFT target image.
So, you can see why Tubb is so smart: He just reaches for the tape and avoids all of this complexity (his eye must go back-and-forth between the target and front sight, many times), and spends his mental energy on the perfect shot, and not battling his visual system. But, in less visually demanding disciplines, like archery, it becomes possible to "easily" understand which images that you need to aim with, without much problem, and there is not rapid change of the sight picture (going between two targets to two front sights to two targets, and so forth).
If you're interested in vision, do a search on "Brock String" (a fantastic vision training tool that you can make for free), or look into "sports vision therapy."
Note: If you are aware of some literature on the subject of aiming with both eyes open, please be so kind as to send me an email!
Kleanthes Koniaris, email.