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Close focus distance is the closest point that the binocular can focus on. This distance varies from about 0.5 to 30 m (2 to 98 ft), depending upon the design of the binoculars. If the close focus distance is short with respect to the magnification, the binocular can be used also to see particulars not visible to the naked eye.
It is this too-close focussing of the eyes that is instrument myopia. Accommodating to a distance nearer than 6 meters makes the ciliary muscles work harder than they need to, leading to fatigue. [5] Some binocular microscopes are designed so that the vergence of the eyes is correct for a 6-meter viewing distance.
The distance between the two eyes on an adult is almost always 6.5 cm and that is the same distance in shift of an image when viewing with only one eye. [17] Retinal disparity is the separation between objects as seen by the left eye and the right eye and helps to provide depth perception. [ 17 ]
This distance is referred to as the back vertex distance, or BVD on a prescription. Since this property affects the available eye relief of any binocular or other optics used, (telescopes, microscopes, etc.) it should be borne in mind at the eye testing stage. The matter should be discussed with the optician, though the only realistic way of ...
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A corrective lens can be used to correct hyperopia by imaging an object at the typical near point distance D onto a virtual image at the patient's actual near point, at distance NP. [2] From the thin lens formula, the required lens will have optical power P given by [3] [4].