142 Double-Image Focusing Spot
The viewfinder of a rangefinder camera is an optical system that is not connected to the lens. When looking into the viewfinder eyepiece you see through the viewfinder window directly opposite. With an SLR, by contrast, you see directly through the picture-taking lens.
Next to the viewfinder eyepiece—used for framing the image—early rangefinder cameras had a separate rangefinder eyepiece for focusing the image. But it is fairly rare to find a camera with two separate eyepieces, as they are much more commonly combined into one range-viewfinder eyepiece. The view from the rangefinder is superimposed into the center of the viewfinder via a series of prisms or mirrors, allowing you to compose and focus your picture in one go. The shape seen from the front in the center of the rangefinder window is the shape that you see as a double-image focusing spot in the range-viewfinder eyepiece.
To get an idea of the mechanics of the combined range-viewfinder system, if you cover the rangefinder window from the front while looking through the eyepiece, you will see the focusing spot disappears from the viewfinder. Likewise, if you cover the viewfinder window while looking through the eyepiece, you will see only the focusing patch.
In most cameras with a combined range-viewfinder eyepiece you will also see frame lines that indicate the area that will be recorded on your film. These may move when focus is adjusted to compensate for parallax. If the camera has a built-in light meter, there will also be an exposure scale with a meter needle.