Parallax describes the difference between what is seen in the viewfinder and what the lens records on film. This difference is caused by the separation between the viewfinder and the picture-taking lens on a rangefinder camera.
This means that rangefinder cameras aren’t fully “what you see is what you get” because the image is framed with the viewfinder while the photo is taken with the lens, which is typically a few inches below and to the side. This parallax error is most evident at close distances and is the reason that most rangefinder lenses cannot focus closer than 3 feet. Any distance closer than this would differ dramatically from what the lens sees, resulting in unpredictable framing of photographs. In the example to the left (shown reversed from the front of the camera) frame 1 illustrates what is seen in the viewfinder, and frame 2 shows what is seen by the lens, highlighting the discrepancy between what is framed and what is recorded with no compensation for parallax.
Parallax Compensation
To combat parallax error, some rangefinder cameras have automatic parallax compensation, where frame lines in the viewfinder move to reframe the image as the focus is adjusted. At close ranges, the frame lines in the viewfinder will shift toward the optical axis of the lens. Anything outside the frame lines will be cropped. In some cases the area of the frame even contracts very slightly to match the narrowing of the picture angle as the distance between lens and film is increased.