Most of today's binoculars will allow you to separately adjust the focus on one eye (usually the right eye) with a diopter. This is done to compensate for differences between your eyes so you see the clearest image possible through your binoculars.
To make the diopter adjustment, pick a distinctive object to focus on that is beyond the close focus distance of the binocular, but not too far out in the distance. Initially, come to a sharp focus on the object using the center focus of the binocular (with the diopter-corrected eyepiece side blocked off or that eye closed).
Once you've got the first eye focused as sharply as possible, close it and, then, open your other eye (the one on the diopter-corrected side). Carefully, without moving the center focus of the binocular, see if you can improve the image sharpness through this eye by slowly moving the diopter adjustment back and forth. Once you've found the setting which gives you the sharpest image, note it and leave it there -- you're done. From this point on, you simply use the center focus to adjust both eyes while viewing.
Note: You should properly set the diopter adjuster when you first use your binocular (make note of your setting). Check the diopter setting on your binocular every so often to be sure your eyes haven’t changed. Always check it whenever you share your binocular with someone else.
To help you better understand diopter adjustment, we invite you to view our 3-minute video, "Understanding Binoculars: Fit and Focus."