Classic old lens adaptation to mirrorless cameras: Flange Focal Distance (FFD) physical mechanism and infinity focus

1. Flange Focal Distance (FFD) and the physical laws of non-reverse switching

Flange distance refers to the precise physical vertical distance between the metal mounting plane of the camera mount and the photosensitive surface of the sensor (or film):

• Traditional SLR camera: Because a mechanical reflector that bounces up and down at a 45-degree angle must be accommodated inside the body, the flange distance is extremely long (such as 46.5mm for Nikon F mount, 44.0mm for Canon EF mount, and 45.46mm for M42 screw mount).
• Modern mirrorless mirrorless: The reflector is completely removed, the sensor is extremely close to the mount, and the flange distance is extremely short (such as Sony E mount only 18.0mm, Nikon Z mount only 16.0mm, Canon RF mount only 20.0mm, Fuji X mount only 17.7mm).
• The golden rule of universal transfer:
  $$\text{Only: The native flange distance of the adapter lens} > \text{The flange distance of the body mount}$$
  By making a hollow metal extension tube (adapter ring) with a length exactly equal to the difference between the two, the focus of the old lens can be accurately projected on the mirrorless CMOS surface! Therefore, the short flange distance mirrorless camera is the "ultimate universal digital back" that can be used to transfer all old lenses in the history of human photography.

2. Adapter ring thickness tolerance and “out of focus at infinity/super infinity”

• The adapter ring is thicker (even within 0.1 mm): It is equivalent to placing a miniature close-up adapter ring behind the lens! The lens can capture close-up flowers clearly, but when the focus ring is turned all the way, it cannot focus on a distant mountain building dozens of meters away (infinity is defocused and blurred);
• The adapter ring is made thin (the mainstream industry is deliberately biased): Many excellent adapter ring manufacturers (such as Leina and K&F Concept) often deliberately design the adapter ring to be 0.05 - 0.1 mm thinner than the theoretical value in order to cope with the aging tolerance of different old lenses. The consequence is that the focus ring has reached focus in advance before it reaches the nominal "∞" scale line (called super infinity). This is a normal phenomenon and you only need to manually adjust it by looking at the screen.

3. Assisted focus tool: peak color and 100% local pixel amplification

Old manual lenses do not have an autofocus motor and must rely on purely manual rotation of the focus ring:

1. Focus Peaking: The in-camera high-contrast detection algorithm will plate a circle of bright red, yellow or white high-frequency dotted lines on the edge of the object in focus, making it extremely efficient and intuitive when sweeping the streets and capturing photos.
2. Focus Magnifier: Set the custom button on the body (such as C1 or AF-ON) to [Focus Magnifier]. The moment you press the button, part of the viewfinder instantly magnifies the 5x or 10x pixel-level view. Rotate the focus ring until the eyelashes or brick patterns are extremely sharp and press the shutter. The hit rate can reach 100%.

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