Even the best (read extravagantly expensive) binocular on the market will give you a fuzzy view of the world until you correctly set the diopter ring. This adjustment, typically found on the right eyepiece or center-focus knob, fine-tunes the binocular settings to compensate for any visual differences between your eyes. Set properly, a diopter will not only max out the performance of a high-end binocular but even boost the sharpness of budget glass.
[A] Set the diopter ring to the center of the adjustment scale. This is most likely marked with a zero; on some models it might be indicated with a hash mark or some other symbol. Cover the right lens barrel with a lens cap or duct tape.
[B] Pick an object in the middle distance zone, about 50 yards away. Keeping both eyes open, move the focus ring until the image is at its sharpest. Although you are focusing only with the left eye, keep both eyes open and relaxed. Do not squint.
[C] Switch the lens cap or duct tape to the other lens barrel. Look at the same object, and turn the diopter ring (see inset) to bring the object into sharp focus. Make sure the focus knob doesn’t change. Keep both eyes open; do not squint.
[D] Remove the lens cap or duct tape and look through both lens barrels. The image should remain sharp. Make a note of the diopter-ring setting, or place a small dot of fingernail polish on the correct adjustment. If your visual acuity changes during the year, you may need to reset the diopter.