Dioptric correction - Binoculars
The dioptric correction feature, which is (normally) found on the right ocular, is used to adjust the binoculars to accommodate any eye defect. To set the focus for the left eye, simply turn the centre focus wheel to focus on an object about 100 metres away.
It is easier to do this if you cover the right side of the binoculars. Once you have done this, cover the left side of the binoculars and use the dioptric correction feature to set the focus for the right eye.
It is only necessary to adjust the dioptric correction once. You can simply use the centre focus wheel to adjust the focus for other distances.