Binoculars Explained : Phase Correction

By design, roof-prism binoculars split the light entering the barrels into two separate paths. After passing through the objective lens, the light waves are reflected off the surfaces of the roof prism and are split into two out-of-phase beams of light. Light reflected from one roof surface is 1/2 of a wavelength shifted from the light hitting the other roof surface, sometimes referred to as "out of phase" or "phase shift".

Although the light waves are subsequently forced back together when they reach the viewer's eye, this phenomenon results in reduced contrast and image resolution. This effect does not occur in Porro prism designs. A manufacturer can apply a thin coating on the roof prism surface of the binocular which forces the light beams back into phase, thus improving image quality and contrast, creating a sharper view, especially noticeable when viewing fine detail.