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6. Eye relief (1 Viewer)

The exit pupil (the circle of light that carries the image) created by binoculars is located around 15-18 mm behind the eyepiece. This distance from the exit pupil to the top eyepiece lens is referred to as the eye relief. To use the binoculars comfortably, without distracting effects sometimes called ‘blackouts’, and to ensure the full field of view can be seen, the pupil of the user should be at exactly this distance from the top eyepiece lens.

Incorrect operation, for example holding the binoculars at an angle to the user’s eyes, can eliminate the benefit of a large field of view. Also, if the eyes are too close or too far away from the eyepiece, blurry edge shadings (‘blackouts’) occur. In the right position, the field of view is not only complete but also has a clearly defined edge.

To ensure that the user’s pupils are not too far from the binoculars, spectacle wearers should normally screw the eyecups down to their lowest position (or if soft rubber eyecups are fitted, fold them down) so that the spectacles do not add too much distance between the binoculars and the user’s eyes. Users who do not wear spectacles should normally use the binoculars with the eyecups in their highest or unfolded position.

However, this is an approximation only. Without spectacles, everyone places the binoculars at different distances from the eyes. Many users literally press the binoculars into their eye sockets, others hold them far away and barely touch the eyecups with their eyebrows. Spectacles also come with different frames and lenses and wearers use them in different positions on their nose, which changes the position of the binoculars relative to the user’s eyes. Therefore, the eyecups on the latest ZEISS binoculars can be adjusted over several click-stops so that everyone can find the setting that best suits his/her needs. This is a key point that is often neglected in practice.

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To get the total field of view, the user's eye (eye pupil) has to be located at the same place as the exit pupil of the optical device. The distance from the centre of the last lens to that plane is called eye relief.
 

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