120 Film Backing Paper Light Leakage (Backing Paper Frame Number Bleed) Causes and Prevention

1. Backing paper ink bleed (Chemical Bleed): chemical photosensitivity caused by non-light

Many photographers think that the backing paper numbers printed on the film are due to light leakage from the camera's red window. However, even on a completely sealed Hasselblad back or Mamiya 67 body without a red window, clear "backing paper ghost numbers" will still appear on the film:

• Physical roll structure of 120 film: 120 film consists of a strip of photosensitive film and a strip of light-shielding backing paper coaxially and tightly wound on a plastic axis. The emulsion layer of the film and the printing layer of the backing paper are tightly bonded together.
• Volatile organic solvents (VOC) and reducing gas penetration:
• The back of the backing paper is densely printed with frame number numbers, arrows and stripes. When some batches of backing paper ink are incompletely cured or get damp, the solvent molecules in the ink slowly release volatile gases.
• These chemical gases penetrate the bottom layer of the backing paper and directly react with the attached silver halide emulsion for spontaneous chemical reduction reaction (Chemical Sensitization).
• Even if it is left in an absolutely dark box for several months, the unexposed emulsion will be chemically reduced to a latent image at the point of contact with the ink. After development, the reverse black printed numbers will be accurately displayed!
• Kodak once recalled a batch of T-Max and Tri-X films due to the backing paper supplier changing the ink formula, causing extensive backing paper penetration.

2. Classic camera red window light leakage: the ultimate nemesis of panchromatic film sensitivity

This is the mechanical and physical light leakage of early old red-window cameras (such as Holga, old Fujifilm double-lens reflex cameras, and early Voigtlanders):

1. The original historical premise of Red Window (the era of color-blind films and orthochromatic films):

• Most of the early films in the early 20th century were color-blind films (only sensitive to blue light) or orthochromatic films (sensitive to blue-green light, not sensitive to red light at all). Therefore, it is absolutely safe to open a small red window on the back of the fuselage and let the red light pass through the backing paper to observe the frame number.

2. The tragedy of modern panchromatic film:

• All modern black and white films (such as Kodak Tri-X, Ilford HP5+) and color negative films are panchromatic films (the photosensitive range covers all visible light from 380nm to 700nm, and is extremely sensitive to red light)!
• If strong direct sunlight shines on the red window of the camera for a long time, infrared light and deep red light will directly penetrate the weak yellow/black backing paper fibers, leaving a dazzling overexposed light leak spot and digital silhouette in the center of the film.

3. High temperature and high humidity storage accelerates the adhesion and hydrolysis of the backing paper and the drug film.

The hot and humid environment is the catalyst that triggers chemical penetration and physical adhesion of the backing paper:

• Gelatin emulsion absorbs moisture, expands and softens: When the relative humidity of the environment exceeds 70%, the gelatin protective layer on the surface of the film absorbs water, softens and becomes sticky.
• Backing paper carbon black falls off and embeds: The gelatin layer close to the backing paper firmly adheres the printing black toner on the surface of the backing paper. At the moment when the film is being advanced and pulled, not only the digital imprints penetrate into the film, but also the emulsion layer is partially torn and strained, and white degumming scratches appear scattered on the film.
• Summer car trunk baking disaster: The temperature in a closed car is as high as 60°C, which can cause irreversible chemical heat diffusion of the backing ink of the entire roll of 120 film within a few hours.

4. Professional practical guide to eliminate backing paper marks

1. Blackout tape seal for old red window camera:

• Paste a thick piece of pure black electrical insulation tape (matting and shading tape) on the outside of the red window of the old camera.
• Only when the film is being filmed and changed, gently peel off the tape in the shadow to take a look at the frame number, and then reattach it immediately to seal it. It is strictly prohibited to expose it to the hot sun.

2. Fresh film should be purchased and taken immediately. Be careful not to leave it at room temperature for a long time:

• After the 120 film is opened in the plastic foil sealed bag, it must be photographed within 1 to 2 weeks and sent for processing as soon as possible. It must not be stored in the back for half a year.
• Unopened film must be stored in a dry box or frozen in a sealed bag with silica gel desiccant (-18°C). The chemical diffusion movement of molecules is completely frozen at extremely low temperatures.

3. Unwinding, pinching and light protection:

• When taking out the 120 film after shooting, you must pinch the film tightly along the take-up shaft and roll it, and immediately lick the original self-adhesive seal to lock it tightly. If the roll is loose (Fat Roll), light will enter through the gaps on the sides, causing light leakage around the edges.

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