120 film cartridge dark box light-leak-proof velvet and uneven tension of the pressure plate causing soft focus and scratches

1. The physical nightmare of 120 film flatness: suspended tape without a cassette

Unlike 135 film, which is tightly confined by a metal iron cylinder, 120 film has inherent physical instability:

• Exposure window with huge suspended span:
• 6x6 format negatives with net dimensions up to 56mm × 56mm;
• The back of the film is tightly attached to a thick paper light-shielding backing paper, and the entire film spans the huge metal exposure frame like a suspension bridge inside the fuselage;
• Core mission of Metal Pressure Plate:
• A rectangular metal pressed plate made of high-hardness stainless steel and mirror-polished is installed on the front of the back liner;
• Four precisely calibrated high-rebound helical micro-leaf springs (Leaf Springs) are installed at the four corners behind the pressing plate;
• Its sole mission is to firmly flatten the film on the inner wall of the exposure track from behind the film backing paper with absolutely uniform and constant positive pressure, ensuring that every microcosm of the 56x56mm full frame sleeps exactly on the absolute conjugate focal plane of the Zeiss lens!

2. The double disaster caused by uneven tension of the pressing plate: unilateral virtual focus and surface scratches

After decades of high-intensity use, the back springs and metal plates are prone to mechanical lesions:

1. ‘Unilateral softening and defocusing (Uneven Focus)’ caused by spring fatigue and weakness:

• If the two springs on the left or upper side of the back liner experience metal fatigue and lose their elasticity;
• The film pressing torque is unbalanced, the film is pressed tightly on one side of the exposure frame, while the other side naturally arches 0.2~0.5 mm inside (the film itself naturally has curl memory tension);
• Film Syndrome: In the developed film, the hair of the characters on the right side of the picture appears to be unstoppable, while the buildings and scenery on the left side show a strange "false focus", which is extremely dazzling under large apertures.

2. ‘Penetrating scratches (Scratches)’ caused by burrs and foreign matter embedded in the pressing plate:

• If the mirror chrome plating layer on the surface of the pressing plate is worn and peeled off, microscopic metal barbs appear, or a tiny sand quartz particle is caught in the gap;
• Every time you shake the handle to forcefully advance the film, the film backing paper and even the film itself are pulled under huge friction pressure, and the foreign matter cuts an irreversible horizontal white bright scratch deeply on the surface of the film like a carving knife! The entire roll of film was scrapped on the spot.

3. Dark Slide Seal Leak on the back of Hasselblad

The most notorious expendable failure of the Hasselblad A12 back is this:

• Microscopic sandwich structure of anti-light leakage grating:
• When pulling out the metal light shield (tofu knife), the socket must be physically locked instantly to cut off the light;
• The interior is tightly clamped by a set of ultra-thin stainless steel resilient metal springs (Foil Spring) and a dark black special acid-free polyurethane velvet light-blocking sponge;
• Hydrolysis and powdering of the sponge leads to beam injection:
• After 10 to 20 years, the internal micro-sponge will inevitably hydrolyze and pulverize and lose its expansion and resilience;
• When pulling out the tofu knife to shoot, strong direct sunlight shines into the inside of the film compartment along the gap;
• Burn a series of classic white/orange-yellow diagonal light leak spots that erode from the edge of the film into the frame on the edge of the film! The sealing kit must be replaced regularly.

4. Practical guide for back maintenance and self-replacement of anti-light leakage kit

• Self-repair steps for Hasselblad light leak sponge:

1. Use a No. 0 precision screwdriver to unscrew the two tiny screws on the side of the back slot and remove the metal guard plate;
2. Use tweezers to pick out the old metal shrapnel and powdered sponge that have turned black and softened inside;
3. Replace with a brand new original/auxiliary factory customized light-blocking kit (including a high-elastic stainless steel compression spring and a high-density light-blocking foam);
4. Reinstall the screws and tighten them. Repeatedly insert and pull out the tofu cutter several times to test that the damping is smooth and tight, and 100% eliminates edge light leakage.

• Self-inspection and waxing maintenance of sheeting plate flatness:
• Take out the back liner, press the four corners of the pressing plate with your fingers, and confirm that the spring rebound damping feel in the four corners is absolutely even and synchronized;
• Check the mirror surface of the pressing plate with a strong light. If there are stubborn glue stains, wipe them with absolute alcohol;
• Senior technician’s trick: apply a very small amount of pure natural Carnauba Wax on the surface of the pressing plate and polish it thoroughly with deerskin. The friction coefficient plummets by 80%, the film passing damping is as light as silk, and the film is completely insulated from scratches!

---