, also known as theater binoculars
or Galilean binoculars
, are compact, low-power optical magnification devices, usually used at performance events, whose name is derived from traditional use at opera
performances. Magnification power below 5x is usually desired in these circumstances in order to minimize image shake and maintain a large enough field of view
. A magnification of 3x is normally recommended.
Unlike binoculars, which are a pair of matched, folded terrestrial telescopes, opera glasses use Galilean optics, having a convex objective but a concave eyepiece. This allows the device to produce an upright image with a short length, although with some loss of image quality and angle of view.