eye relief

Definition: The distance between the rear of the eye-piece to the focal plane of the eye-piece optics. * The distance measured on the optical axis from the vertex of the read of the eye lens to the exit pupil. * Also known as eye distance.
The longer, the easier
Eye relief is an important measure of comfort for the user of an optical equipment, from binoculars, microscopes to cameras. Too tight a distance, and the user must align the eye too closely to the equipment for comfort -- which makes it difficult for spectacle wearers. Longer distances are more difficult to design but benefit most users; but eye relief can also be too long for comfort.

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