Diopter correction
From The Glossary of Digital Photography
An adjustment on or to the optical eyepiece of a camera designed to adjust the eyepiece so that the photographer can use the camera without glasses. If the camera viewfinder has a built-in adjustable viewfinder, the diopter correction is adjusted by the photographer to his or her own vision. If the camera does not have a built-in adjustable viewfinder, a separate diopter corrective lens is added to the viewfinder to accomplish the same purpose. This correction changes the focus, but not the magnification, of the viewfinder. See also diopter
