Diopter

One of the two eyepieces of a pair of binoculars is fixed and one is adjustable (usually the right one), called the diopter. By adjusting the diopter, the difference between your eyes can be accommodated. The way to do this, is as follows: Close your right eye (or put the lid on the objective lens of that side) and focus on an object some 10 meters away with your left eye, adjusting the central focusing mechanism. Now do the same with the left eye and focus on the same object, but now adjusting the diopter. Do not touch the central focusing mechanism at all while doing this.  Now the binocular is calibrated for your eyes and from here only the central focus wheel/toggle has to be adjusted when focusing on an object. Some binoculars have locking mechanisms to prevent the diopter setting from being turned by accident once it had been set.

Some binoculars have an adjustable diopter for each eye – not only for the right eye. Invariably these binoculars are of the “auto focus” binoculars type. Their focus has been set at the factory and the viewer does not need to adjust the focus at all; the object is always in focus from about 20 feet to infinity. All the viewer has to do, is to calibrate the binoculars for his personal use by adjusting each diopter and locking in the settings right  at the beginning.

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