A binocular is actually two identical telescopes (prisms added) mounted side-by-side and perfectly aligned to let the viewer view distant objects with both eyes. The alignment, commonly referred to as “collimation“, involves not only this mechanical alignment, but also the optical elements along the binocular optical axis. Both the physical and the optical alignment have to be perfect; if not, double images will result, which will eventually cause eyestrain and headache. If the alignment is “out”, the image will not merge. Fortunately some binoculars do have adjustment screws enabling you to collimate the prisms, but this should only be attempted by a skilled person.

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