The diopter is a ring usually located directly underneath the right-side eyecup. Where the center focus wheel adjusts the focus of each barrel in tandem, the diopter individually focuses only the right side to account for people with different prescriptions.
To set your diopter, start with your right eye closed, looking through only the left barrel of your binocular. Using the center focus wheel, adjust the focus until the image through the left side with your left eye is perfectly clear. Once the left side has been made clear using the center focus wheel, close your left eye and open your right eye. Now, use the right-side diopter underneath the eyecup to adjust the focus for your right eye individually (if necessary). Once the right side has been made perfectly clear to your right eye, you can open both eyes and use the center focus wheel from then on. Now, both eyes should focus together, even if they are different prescriptions.
