Medium format Rollei twin-lens (Rolleiflex) viewfinder lens and camera lens parallax compensation (Parallax Compensation) mask linkage mechanism

1. The physical Achilles heel of dual-lens reflex cameras: vertical parallax

The reason why twin-lens reflex cameras cannot achieve the absolute "what you see is what you get" like SLR cameras is determined by their optical architecture:

• Up and down vertical optical axis distance (Optical Axis Offset):
• The Heidosmat 80mm viewfinder lens above is responsible for reflecting light onto the frosted glass top-view focusing screen;
• The Planar or Xenotar 80mm photographic lens below is responsible for projecting light onto the 120 film;
• There is a physical fixed distance between the central optical axes of the two lenses in the vertical direction of approximately 42mm to 45mm;
• Perspective slice at close range:
• When shooting infinity scenery, the 45mm spacing is insignificant in the vast world, and the parallax can be ignored;
• But when you get close to the subject and shoot a half-length portrait at an object distance of 1 meter, the bottom edge of the picture seen by the viewfinder is nearly 5 cm higher than the picture actually captured on the film! If the photographer composes the picture according to the edges of the viewfinder screen, the resulting film will inevitably lead to the embarrassing tragedy of "the top of the head being cut off and the chin leaving too much white space."

2. Rollei’s top engineering miracle: Moving Mask automatic mechanical linkage

Rollei-Werke engineers invented the purely mechanical automatic parallax compensation system that is still amazing today in the 1930s:

1. Cam rack and front group focus plate linkage:

• When the photographer turns the focus handwheel to push the front lens plate forward (aiming at close range);
• A precision link behind the lens plate pushes a set of tiny levers and gears inside the body;

2. The "sliding curtain" below the viewfinder focusing screen:

• Directly under the frosted glass focusing screen, there is an ultra-thin black matte metal moving frame mask (Moving Frame Mask) hidden;
• As the lens gradually extends forward to focus at the minimum focusing distance of 1 meter, this metal shield will automatically slowly push down and slide out a few millimeters from the top of the viewfinder screen**!
• Automatically blocks out the upper false blind area that can be seen by the viewfinder lens but cannot be captured by the photographic lens, forcing the photographer's composition safety field of view downwards, so that the frame range displayed on the focusing screen remains 100% consistent with the photographic lens below!

3. Breaking through the 1-meter close-up limit: Optical prism parallax correction of Rolleinar macro adapter

The native minimum focusing distance of a dual-lens reflex camera is only about 1 meter. To take 30cm-50cm close-ups of jewelry or flowers, you must use the official Rolleinar macro lens set (Rolleinar 1/2/3):

• Two-piece precision optical set:
• Lower lens (photography lens): install an ordinary high-quality plano-convex close-up convex lens;
• Upper lens (viewing lens): Not only is a convex lens installed, a whole optical wedge-shaped prism (Prism Compensator) with precise calculation of angles is glued on top of the lens!
• The light path of the prism is forcibly refracted downward:
• The wedge-shaped prism uses the law of refraction of light to forcibly deflect the light axis of the upper viewfinder lens downward by several degrees;
• Make the optical axes of the upper lens and the lower lens accurately intersect and converge at one point on the object plane at 30cm or 50cm;
• Red Dot Up installation rule (Red Dot Up):
• An eye-catching red alignment dot is engraved on the outer ring of the Rolleinar upper lens;
• The red dot must be strictly facing the 12 o'clock direction directly above during installation! Make sure the prism deflects light downward. If the red dot is carelessly installed on the side or below, the viewing parallax will be magnified several times, causing the viewfinder and film to be completely misaligned.

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