The eyepiece on the one side of a pair of binoculars – usually the left hand side – is fixed and one on the other side – usually the right hand side – is adjustable; this is called the “diopter”. Most brand names have the diopter in the right eyepiece; Steiner has it in the left eyepiece. By adjusting the diopter the difference between the focusing or viewing ability of your two eyes is accommodated. Some binoculars have locking mechanisms to prevent the diopter setting from being turned by accident once it had been set.

Some binoculars (from the “auto focus” binoculars type) have an adjustable diopter for each eye – not only for the right eye. Please consult my article on Auto focus binoculars for a discussion of this category of binoculars.

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