First, extend the eye cups. They are meant to block light as well as position the lens away from your eye. If you wear glasses, read Using Binoculars if You Wear Glasses. Then, adjust the binocular barrels, making sure they are in a position matching the distance between your eyes to give you the clearest, most comfortable view. (This is called interocular distance.)
Next, you’ll need to adjust and set the diopter. The diopter is usually found on the right barrel and should be set at the zero point before you begin adjusting it to your eyes. Choose a fixed object to focus on. Something 30 to 50 feet away with writing on it (such as a sign) works well. Don’t look through a window if you can help it, because it adds another optical element to worry about. Cover the lens related to the diopter (most often the right side) with the lens cap, or just hold your hand in front of it. Keeping both eyes open, adjust the focus using the center dial. Once the view is sharp, cover the other lens but this time focus using the diopter. You may need to fine-tune your focus by adjusting the central focus again and maybe tweaking the diopter a second time. Finally, check your view by looking through the binoculars normally. Once you’re satisfied that the focus is clear, you won’t need to set the diopter again, unless you switch corrective lenses. From now on, simply use the central focus. (Some binoculars don’t have central focus; most birdwatchers don’t have time to mess with two eyepiece adjustment dials.) If the diopter has a locking mechanism, lock it at this point.
If you have zoom binoculars, perform the focusing steps at the highest power setting.
Historically, binoculars have come in two focusing arrangements: Independent focus means each side must be focused individually, while central focus allows the viewer to make use of a central focusing wheel. Independent, also known as individual, focus is a good design for binoculars intended to be used in dirty or wet settings, such as the military, because it’s much harder for dirt and moisture to penetrate the mechanism.
Also on the market are fixed-focus (also known as focus-free, self focusing or auto focus) binoculars, which reach a compromise by having a wide depth of field while focusing at what would be the middle range of comparable manual-focus binoculars.
