en.wikipedia.org/wiki/Binoculars
Porro configuration which is the traditional arrangement resulting in a wide binoculars. The objective lenses are not aligned with the eyepieces. The prisms reflect the light through an "S" shape path to the eyepiece. "Roof prism" binoculars align the objective lenses directly with the eyepieces and are much narrower than the porro configuration. By definition, the magnified image is available to both eyes of the observer.
telescopes, always pointing in the same direction, with the two oculars arranged so that it is possible to look through them using both eyes. Most binoculars have a mechanism for changing the distance between the oculars, to adapt to different observers.
ratio between magnifying power and objective lens diameter. Due to the way binoculars are made, the resulting ratio is the diameter of the final image on the oculars.
pupil diameter, that in dark environments grows to about 7mm. This ratio is also a measure of the brightness of the image. Thus, 10x50 and 8x40 binoculars have the same brightness, although the latter has a smaller image. A smaller image would use only partially the observer's eye, while a larger image would be impossible to see in a single glance. Because of this, binoculars with a magnifying power like 7x50 or 10x70 (close or at the 7 mm ratio) are the best choices for their diameter, giving so to speak the best "bang for the buck".
It is much easier to follow fast-moving objects such as aircraft in flight, water skiers or race horses using binoculars than using a telescope of similar optical power.
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