The diopter adjustment is designed to let you compensate for differences between your two eyes. It can mess up your viewing if it gets turned accidentally.
Here is how to set your binocular's diopter adjustment to match your eyes.
Many people’s left and right eyes differ in their vision. Therefore, binoculars let you alter the focus of one eyepiece with respect to the other. It’s called the diopter adjustment.
For most Porro prism binoculars, like the ones pictured here, the right eyepiece is the one you can individually adjust.
The diopter adjustment is on the eyepiece itself. There's a mark for the center, or zero, position. You can turn the ring to the left or the right of the zero position.
Roof prism binoculars, like the one shown at right, may have the diopter adjustment on the center column. If you’re not clear about where the diopter setting is on your binoculars, consult the manual they came with.
The technique for customizing the diopter adjustment is the same for both styles of binoculars.
Start by placing the diopter setting at the zero, or center, position.
Cover the barrel which the diopter adjustment affects, (which is usually the right side). It's best to use a lens cap. If you haven't got a lens cap, you can use your hand.
But don’t just squint your eye to do this. When the eye is squinted closed, the pressure on your eyeball temporarily changes its shape and makes it focus differently. That can throw your adjustment completely off.
Pick an object in the middle distance. Keep both eyes open and focus the binoculars, using the regular center-focus knob, until the image is clear and sharp. Now the left eye is in focus.
Next, without disturbing the focus we have set for the left eye, we’ll use the diopter adjustment to focus the other eye.
Now switch the lens cap, or whatever you’re using, to the other side.
Looking at the same object as before, and keeping both eyes open again, turn the diopter setting back and forth until the object is in sharpest focus. Be careful not to disturb the center focus.
Now remove the lens cap and look through both eyepieces at once. The image should be in sharp focus in both eyes, and your eyes should feel comfortable while looking at it.
Other binoculars, like this Zeiss roof prism model — may have the diopter adjustment on the center column. If you’re not clear about where the diopter setting is on your binoculars, consult the manual they came with.