Binoculars and Porro prisms to show.

Binoculars and Porro prisms to show. The sets of 45 deg -45 deg -90 deg Porro prisms used in prism binoculars are useful for several reasons. They can take an image from a large lens, and displace it for convenient viewing. This displacement uses total internal reflection (almost 100% efficient), which is better than mirrors. Thus the image will be brighter. Also, the two Porro prism system erects and reverts the image from the objective, eliminating the need for an extra lens, and shortening the path length of the instrument. See figure 1. An image 'F' enters the prism arrangement, and is totally internally reflected, exiting with the F erected and reverted. Each half of a prism binoculars is really a simple telescope. See figure 2. Light enters through the objective (f.l. 19 cm) at point 1. The Porro prisms at point 2 displace the image and erect and revert it, so that the beam now focuses at point 3. The lens (f.l. 4.5 cm) at point 4 and the lens (f.l. 3 cm) at point 5 are both used as magnifiers in the adjustable eyepiece, giving the binoculars a magnification of 7 times (at 1000 yards the field of view is 372 feet).