How to … WHAT???
We should probably start with what the diopter setting is and why it’s so important.
Most binoculars you’d buy for hunting or general wildlife watching are called “center focus.” That means there’s a dial located somewhere between the two eye tubes that changes the focus of both sides simultaneously when you turn it. Quality binoculars also offer a diopter adjustment to compensate for the difference in strength between your two eyes. (Few people have exactly the same visual acuity with both eyes.)
The location of the diopter adjustment varies. Sometimes it’s a ring behind an eyepiece (usually the right). On some high-end binoculars, the center focus knob slides back to become a single lens adjustment. Read the owner’s manual or search on line to determine the location of the diopter adjustment for your particular brand and model of binoculars.
Properly set diopter adjustment is extremely important. If you experience eye fatigue or even headaches after looking through binoculars, even for a short time, improper diopter adjustment is likely the cause. Setting the diopter is simple and fast, and it makes viewing through binoculars much more pleasurable and efficient. Adjusting the setting is best done with a partner who can alternatively cover the lenses of the binocular with a card or piece of dark paper for you, but you can do it by closing one eye at a time if you’re alone.
Pick an object at some intermediate distance like 100 yards or so. I like to sight on something that has a lot of contrast and fine detail—like the dark, bare limbs at the top of a tree against the clear blue, midday sky.
Assuming the diopter adjustment on your binoculars is on the right lens tube, have your partner hold the card tightly against the lens of the right side. Using the center focus adjustment, focus the view so your left eye sees the object perfectly. Take your time and really make certain the adjustment is absolutely spot on for a sharp view.
Keep looking at exactly the same object and take your finger away from the center focus dial. Have your partner now cover the left objective lens. Carefully move the diopter adjustment behind the right eyepiece back and forth (without touching the center focus) to find the sharpest possible view for your right eye on precisely the same object.
You’re nearly done.
Some binoculars allow you to “lock” the diopter adjustment in place. Others simply provide markings for you to note how many “hash marks” you are either plus or minus from zero. I prefer to actually make some kind of physical mark on the setting dial (a dot of White Out half on the dial half on the body works great). That way if I loan the binoculars to someone else for a look and they change the setting, I can quickly come back to the sweet spot for my eyes.
One other tip—especially from one season to the next—be sure to recheck the diopter adjustment. Our eyes change over time (especially for us “veteran” hunters), so you may need to modify the setting from season to season!