To fix a binocular diopter, ensure that both eyes are set to the same number. You can do this by starting with one eye and then opening the other and adjusting the diopter accordingly. Once they’re both at the same number, close one of your eyes and turn the barrel of your binoculars with an open eye until it’s focused as best as possible. Now open your other eye and use it to check if it’s in focus. If not, then repeat these steps with this new open eye until it is.