Available Eye Relief
The eye relief given in product specifications does not always give a realistic view of what a user can expect. Although eye-cups can usually be folded down to allow the spectacle wearer to get closer to binocular eyepieces, there are sometimes lens mountings that do not allow the theoretical eye relief to be obtained. A better measure for those with strict needs would be one that takes account of this available eye relief, the theoretical value less any thickness of the lens' rims. This point can account for confusion in performance and is rarely expressed clearly.
Additionally, when a spectacle wearer orders new glasses, the optician will ask him whether he prefers his spectacles close to the eyes or at some distance. 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 testing the comfort is to try the optical device while wearing the usual spectacles. The optician can however make sure that the BVD is no worse in your new glasses than in the old ones that were used during evaluation.
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