Some binoculars are designed to provide a sufficient field of view even for eyeglass wearers by simply folding back the eyecup so as to place the eye at the correct eyepoint. Otherwise the field of view might be partially shaded.

Adjustment of pupil distance:

 The purpose of this adjustment is to align the exit pupils of the binoculars with the pupils of your eyes. If they are not correctly aligned, you will experience discomfort from having to use only one eye. 
Binoculars offer a wide range of pupil distances so anyone can use Nikon binoculars in comfort. While you look at a distant object, carefully move the binocular tubes downward or upward so that both fields are correctly aligned, forming a perfect circle.

Adjust pupil distance by opening or closing the binoculars.

Pupil distance is properly adjusted.
Pupil distance is not adjusted. 
(The central part might
 be invisible or an eclipse 
might appear at the corners.)

Focusing Diopter adjustment: 

If there is a difference in visual acuity between your left eye and your right eye, you need to make a diopter adjustment. Looking through binoculars without diopter adjustment will not yield a sharp, crisp image because one eye sees things in focus while the ther doesn't. Worse still, you will suffer eye strain. Most binoculars have a mechanism that compensates for different diopters. All prism binoculars have this mechanism.
 

For CF (Central Focusing) binoculars First look through the left eyepiece with your left eye while closing your right eye and turn the central focusing ring to focus on a distant object. Then look through the right eyepiece with your right eye while closing the left eye, rotating the diopter adjustment ring to focus on the same object.

1) Focus with your left eye. When the diopter is adjusted, turn the central  focusing ring to focus on the object.	2) Adjust diopter with right eye.
Central focusing ring

For  (Individual Focusing) binoculars 

You can adjust the diopters of both eyepieces independently.
Focusing and diopter adjustments are performed simultaneously, but a change of object requires turning both rings.

.Focus with your left eye.	Focus with your right eye.