The eye relief of a binocular is the distance from the eyepiece that you need to place your eye in order for all the light from the eyepiece to pass into your eye when the exit pupil of the binocular is the same size as the pupil of your eye. It is the position of what physics textbooks call the "eye ring" and can be defined as the position of the image that the eyepiece forms of the objective lens. At this distance you will be able to see the entire field of view and will have the brightest possible image. Over the past decade manufacturers have become more aware that spectacle wearers will seek out binoculars with adequate eye relief to enable them to see the whole field of view. As with fields of view, several manufacturers tend to be somewhat "optimistic" in their quoted eye relief so, if you need to wear spectacles for observing, you should verify in practice that the binoculars have a suitable eye relief. So that binoculars can be used by people both with and without spectacles, they will have eye cups that are either twist-down or fold-down to enable the correct positioning of your eye (Figure 3.1). As eye relief increases, it becomes increasingly difficult to position your eye precisely behind the eyepieces. This can exacerbate the kidney bean effect if it is present.
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