Rangefinder film camera yellow spot focus coincidence (Rangefinder) working principle and body infinity fine-tuning calibration

1. Optical triangulation geometric ranging mechanism of double-image coincidence on the paraxial axis

Unlike an SLR camera that observes a matte screen directly through the lens, a rangefinder camera is equipped with a sophisticated optical triangulation system on the top:

• Dual window structure: There is a main observation viewfinder window on the left side of the camera, and a small secondary ranging window a few centimeters away (Baseline Length) on the right side;
• Semi-transparent prism and rotating reflector: The light from the secondary view window passes through a linkage reflector and is projected in the center of the main viewfinder, forming a translucent bright yellow square light spot (yellow spot);
• Mechanical Linkage Roller (Cam Follower): When you rotate the lens focus ring, the spiral cam (Helicoid) at the rear of the lens expands and contracts back and forth, tightly against a miniature metal roller link at the 12 o'clock position above the fuselage bayonet;
• Focus moment: The roller linkage pushes the internal reflective prism to rotate. When the virtual shadow in the macula in the viewfinder and the real scene in the main picture are 100% perfectly overlapped into a single body in the three-dimensional space, the triangular ranging formula is closed at the mechanical level, and the lens focus accurately falls on the surface of the film!

2. Two major fault forms of side-axis focus running

1. Horizontal Misalignment: When focusing on infinity (such as a distant mountain or the horizon 500 meters away), the lens focus ring has been screwed all the way, but the virtual shadow in the macula is skewed to the left or right and cannot completely overlap; or when shooting at a distance of 1 meter, the focus is out of focus.
2. Vertical Misalignment/Up and Down Misalignment: The most dizzying problem! The shadow in the macula is a little higher or a little lower than the main frame**, and it cannot be merged even if you move it left or right. Long-term viewing will cause severe eye fatigue.

3. Practical operation of precision adjustment of Leica M body (taking Leica M4/M6 as an example)

• Tool preparation: Micro flat-blade precision screwdriver, hexagonal eccentric adjustment wrench, red laser pointer or ultra-distant night view lighthouse 1000 meters away.
• Step 1: Horizontal infinity calibration (adjust eccentric bolt):
• Remove the camera lens and observe the elastic focus wheel at the 12 o'clock position just above the mount;
• There is a micro eccentric screw in the center of the roller. Use a special tool to rotate the eccentric bolt very slightly (test the amplitude very slightly in 1-2 degrees), mount a calibration lens and aim it at infinity until the left and right sides of the macula absolutely coincide.
• Step 2: Vertical height coincidence calibration:
• Leica M3/M4/M6 There is a very small round cover decorative screw hidden right in front of the forehead (located between the viewfinder window and the rangefinder window);
• Use a rubber screwdriver to unscrew the decorative cover to reveal the micro-vertical mirror adjustment screws located deep inside;
• Turn the screw clockwise or counterclockwise, and when looking through the viewfinder, the shadow in the macula will translate up and down until it is absolutely flush with the horizontal line of the main field of view. After the calibration is completed, apply a drop of reversible thread glue to reset the outer cover.

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