Binoculars glossary Focusing

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Just like any other optical system, binoculars also need to be adjusted to focus on an object a particular distance away. An image is sharp when an abrupt transition in the subject is shown as an abrupt transition in the image. Or to put it more simply, a point in a subject must be shown as a point in the image and not as a circle. To achieve this, there are various focusing systems possible. In the centre focus system, both halves of the binoculars are focused by turning just one wheel. With the aid of the dioptric correction feature, which is generally located on the right ocular, the binoculars can be adjusted to correct an 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.

Besides binoculars with a centre focus system, there are also binoculars that boast an individual focus system (BIF model). With these binoculars, it is necessary to adjust the focus to accommodate each eye individually. This does seem to be a more cumbersome procedure but it does have the advantage that the binoculars can be constructed more sturdily and that they can be made extra watertight. That is why water sports enthusiasts, among others, opt for binoculars with an individual focus system. In practice, these binoculars are set to infinity so that objects from 7 metres away to infinity appear sharp without requiring any re-adjustment of the focus.

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