Rangefinder Camera Macula Double Image (Rangefinder Alignment) Horizontal and Vertical Mechanical Calibration Guide

1. Rangefinder double-image geometric optics

Rangefinder cameras (such as Leica M series, Voigtlander Bessa, Canon P) measure the subject distance through two optical windows on the top of the body that are a few centimeters apart:

1. Main viewing light path and secondary ranging light path:

• Main window on the right: direct light path, built-in frame frame group and semi-transparent and semi-reflective beam splitter;
• Left side secondary window: The interior contains a rotatable reflective prism (or reflector) that swings in conjunction with the lens focus ring.

2. Triangulation:

• When the lens focus ring is rotated, the copper cam (Cam) at the rear of the lens pushes against the roller ranging arm above the body bayonet.
• The swing arm drives the secondary prism to rotate slightly, projecting the golden or yellow image sampled by the secondary window onto the square area (yellow spot) in the center of the viewfinder.
• Double image overlapping focus: When the main framing image and the yellow virtual image completely overlap in the horizontal and vertical directions, the actual focus plane of the lens falls exactly on the subject.

2. Two common macular misalignment diseases and causes

Due to mechanical drop impact, temperature difference, thermal expansion and contraction, or lubricating oil drying up over time, there are two most common deviations in the rangeshaft mechanism:

1. Vertical Misalignment:

• Phenomenon: The virtual macular image in the center of the viewfinder is a little higher or lower than the main image. No matter how you turn the focus ring, it is always misaligned up and down, making it easy for you to feel dizzy and dizzy.
• Impact: Although it usually does not affect the actual distance judgment before and after focusing, it seriously interferes with the viewing experience.

2. Horizontal Infinity Misalignment:

• Phenomenon: When the lens focus ring is screwed to the infinity $\infty$ hard limit, and it is aimed at a distant iron tower, the horizon or the moon 1000 meters away, the macular virtual image and the main image are still separated in the horizontal direction (the separation indicates that the focus distance indicated by the lens is inconsistent with the judgment of the body rangefinder).
• Fatal impact: Resulting in serious out-of-focus and out-of-focus shooting in all close-up shots with large apertures!

3. Leica M series (taking M3 / M4 / M6 as an example) standard calibration method

1. Prepare high-precision tools:

• Special side-axis adjustment eccentric tool (or watch screwdriver with extremely fine flat mouth 1.0mm~1.4mm);
• Blowing balloons and high-transmission flashlights;
• Test target: A lightning rod 1000 meters away on a clear day or a clear outline of the moon at night.

2. Vertical deviation adjustment (located behind the red mark on the front of the fuselage or inside the bayonet):

• Remove the lens, and at the 12 o'clock position just above the fuselage bayonet, you can see a micro eccentric eccentric adjustment screw inside the rangefinder swing arm roller axis.
• Insert a thin screwdriver and very gently turn the screw clockwise or counterclockwise in micron-level increments.
• Observe distant horizontal wires through the viewfinder while rotating: the upper and lower ghost images immediately rise and fall until the wires are completely straight with no height difference.

3. Horizontal infinity adjustment (roller swing arm length adjustment):

• Put the lens back into the body and set the focus ring to $\infty$.
• Aim at the distant moon or lightning rod.
• If the macula has not overlapped yet, remove the lens and adjust the eccentric cam (Eccentric Screw) on the outer edge of the ranging arm roller. Fine adjustment clockwise will make the macula move closer to the inside and stretch it outward counterclockwise.
• When the lens is stuck at $\infty$, the virtual image of the macula and the real image merge into one.

4. Close-up 1 meter accuracy reverse verification:

• Pull out an accurate 1.00 meter tape measure indoors and tape a piece of paper with a high contrast cross focus target on the wall.
• Accurately align the body focal plane mark ($\Phi$) with the zero mark of the tape measure, and the focus target is at 1.00 meters.
• Rotate the lens and check whether the 1.0m reticle of the lens focus ring exactly coincides with the focus reference line, and the yellow spots in the viewfinder completely overlap. If both ends match, the calibration is complete.

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