Editing Binoculars (section)

=== Alignment ===
The two telescopes in binoculars are aligned in parallel (collimated), to produce a single circular, apparently three-dimensional, image. Misalignment will cause the binoculars to produce a double image. Even slight misalignment will cause vague discomfort and visual fatigue as the brain tries to combine the skewed images.<ref>Stephen Mensing, Star gazing through binoculars: a complete guide to binocular astronomy, page 32</ref>

Alignment is performed by small movements to the prisms, by adjusting an internal support cell or by turning external [[set screw]]s, or by adjusting the position of the objective via [[eccentric (mechanism)|eccentric]] rings built into the objective cell.
''Unconditional aligning'' (3-axis collimation, meaning both optical axes are aligned parallel with the axis of the hinge used to select various interpupillary distance settings) binoculars requires specialized equipment.<ref name="books.google.com"/> Unconditional alignment is usually done by a professional, although the externally mounted adjustment features can usually be accessed by the end user.
''Conditional alignment'' ignores the third axis (the hinge) in the alignment process. Such a conditional alignment comes down to a 2-axis pseudo-collimation and will only be serviceable within a small range of interpupillary distance settings, as conditional aligned binoculars are not collimated for the full interpupillary distance setting range.