Double-Image Focusing Spot / Patch (p. 142)
When you look through the rangefinder (or range-viewfinder eyepiece of a rangefinder camera) and adjust the focus, you see two images coinciding or separating within a comparatively bright spot (square, diamond, or rectangle) in the center of the frame. When focus is adjusted, one of the overlapping images will shift from side to side. When the camera is out of focus for a particular subject, a double image appears within this bright focusing spot. When the two images overlap perfectly and fuse into one sharp image, the subject is in focus.