The optical elements of a pair of binoculars consist of lenses and prisms and could be which could be as much as 16 elements in total. Lenses are the optical elements through which the light travel, from the big ones at the front of the binoculars, to the smaller ones in the eyepieces where you put our eyes at the back of the instrument.
In some cases the objective lens at the front is actually a combination of lenses, as in the case of the Kowa XD-lenses: As a matter of fact, you could also get a combination of lenses in the eyepieces, depending on the magnification of the instrument, as can also be seen in the image of the Kowa.
Between the objective lens at the front and the ocular lens at the back in the eyepiece a number of other lenses are positioned: A focusing lens in the case of binoculars using internal focusing (roof prism binoculars) and a few lenses in the eyepiece – two to five, depending on the magnification of the instrument: The higher the magnification, the more lenses in the eyepiece.
In addition to these a crucial optical element will also be found: A pair of prisms per optical tube. Whereas the lenses ideally let through all the light passing through, the prisms ideally reflect all the light landing on them, as can be seen in the diagram above.
The image entering the objective lens at the front is inverted (backwards and upside-down) and the prisms have to rectify this orientation.
Prisms are basically chunks of glass cut in a very specific way, as can be seen in the video clip below (“How Steiner binoculars are made“). They act like mirrors, reflecting the images entering them. They bring the beam of light from the objective lenses closer together and rectify the orientation of the image before passing it on to the ocular lens.
Fast Tube by Casper