It is surprising how many people do not know how to focus binoculars correctly. There are two common focusing systems used in binoculars.
The first is individual eyepiece focus. This system is simple to understand, and easy to manufacture. It also lends itself well to sealed optical tubes, and thus is usually the focusing system used for waterproof binoculars. Individual eyepiece focus means that to focus the binoculars to your eyes, you simply focus the left eyepiece to your left eye and the right eyepiece to your right eye. There is no centrally located focusing mechanism. It is done like this. Look at something in the distance. Close the right eye (or cover the front of the right binocular), and focus the left eyepiece to your left eye. Close the left eye (or cover the front of the left binocular), and focus the right eyepiece to your right eye. You are finished, until you need to look at something at a different distance, in which case you need to repeat the process.
Because individual eyepiece focus is time-consuming, center focus is more common. Unfortunately, very few people understand how to correctly use center focus binoculars.
Here is how it is done. Aim your binoculars at something in the distance. Close the right eye (or cover the front of the right tube), and focus the left side of the binocular to your left eye using the center focus control, which is concentric with the pivot shaft between the binoculars. (Note: the left eyepiece itself does not focus on center focus binoculars.) Next, close your left eye (or cover the front of the left tube), and focus the right eyepiece to your right eye. DO NOT touch the center focus control while you are focusing the right eyepiece to your right eye. Now you are finished. What you have just done is adjust the binoculars for your individual eyes. (Practically everybody's left and right eyes are different.) From now on, you only need to adjust the center focus control when you look at things at different distances.
Center focus is faster and easier to use than individual eyepiece focus, once you have initially set the binoculars for your eyes.
...read the full guide with pics here
It helps a bit to use the word diopter and show where it is.
This guide is incomplete. It discusses how to set up center-focus binoculars for use (i.e. how to adjust the diopter) but doesn't discuss best focusing practice. For example, it is best (more relaxing to the eyes) to focus from too near to the point of best focus rather than from too far to the point of best focus. So when fine adjusting focus, turn the knob the appropriate direction to focus slightly too closely first, then adjust to the point of best sharpness.
--AP