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Message: <blockquote data-quote="typo" data-source="post: 3124484" data-attributes="member: 83808">It means that the eye should be placed 15mm form the eye lens surface. Closer would lead to blackouts and further away reduction in the field of view. The same eye cup design will usually be used for both 8 and 10x models but they will normally differ in eye relief. Quite how far the eyecups will need to be extended to be both comfortable and provide the correct distance to the eye you will need to work out for your own features.David</blockquote>