Light Piping, its not just Blowing Smoke Light Leak
Light piping is unwanted light exposure along the edge of a strip of film, most common at the beginning or at the end of the roll. You will notice the sprocket hole shape leaking through and exposing to the wound film layer under it on the take up spool in your camera. It is often referred to as unexplained fogging, edge exposure or edge flair. Yet it can happen mid roll in the image area of the negatives as well. This can look like skimming light fogged exposure or just areas of black exposure.
Anytime a loading tail is left out of the film cassette, light has the potential to enter and expose the film inside, before or after exposure. Whether camera loaded or in storage.
This especially is true with thin based films, not necessarily because they are thin, but because of what their base is made of, polyester. Polyester acts like fiber optics and transports light. it keeps drawing it in and spreading it around. Although dyes and anti-halation backings are used in tradition films this may not be the case in specialty films that are not commonly cut into short lengths. That dense grey you see on the base of traditional films is the layer that keeps not only film from light piping but also re-exposing from the back. Light passing trough the film will bouncing off the pressure plate then re-exposing your film.
Light piping factors that increase the chance of exposure are:
Base density and type, polyester as mention is almost like fiber optics and transports light
Emulsion type and base, silver is a light receiver so it’s not so much what the emulsion is made of by what kind of anti-halation or light blocking layer is added to the base. Example, Kodak’s HIE (High speed IR) did not have an anti-halation backing, it could re-expose from the pressure plate. It had warning on the box to always handle in total darkness, including loading and unloading your camera. Other B/W films are not opaque backed but have dyes to avoid this. The film bases can be rather translucent and can still be candidates for light piping.
Short exposure rolls, I’ll have more trouble using shorts like 12 or 15 exposures as the film is looser in the cassettes.
Weak or bad seals in your camera, may not be a problem for traditional film can light pipe expose with polyester based films. These films expose mid roll when left long enough if your seals are weak and the camera is in a strong light.
Light piping is cumulative, as it gathers even small amounts of light and will transmit it to layers wrapped below. As it gets additional exposure it will transmit even more fogging to your film.So even in the dimmest of light your film can edge fog with only a few hours of exposure.
I did a test just to see how sensitive these films are. I used my favorite Svema ISO 200. Svema 200 is a thin polyester based film and uses a non-ultra dyed backing. I personally have had light piping with this film in the past. All my rolls were loaded from the same bulk loader, in previously used cassettes and loaded in room light. The cassettes were then placed in, #1, an all black plastic can, #2, a translucent can and #3, no can at all. The #2, translucent was kept in a room with very little light but not totally dark, it had a single lamp lit for only an hour or two a day. The non canned film traveled around in my camera bag, my purse, sat on my desk, had lunch with me etc, but was never in direct light. Except for #3, the test lasted for 10 days. #3 was only with me for a couple of days. They were all processed the same time, developer and tank. The results were, #1, Black can, totally clean and image area blank and no unwanted expossure, no piping, #2, some image fog, some light piping, and #3, is a train wreck!
To prevent light piping you must protect it from even low levels of light. Keep unexposed and exposed, unprocessed film, in a black can or bag until loaded or processed.
Yes this takes a little more thought and care to handle these films if you want to avoid light piping but personally I think these films are pretty amazing and worth the effort.
Leslie Lazenby fell in love with photography when she was given her first camera, a GAF 126, at the age of 10. Her first job in a camera shop with a custom and commercial photo lab turned into a 20-year adventure in film; leading to positions in darkrooms, customer relations, and as head of purchasing. For the past 15 years, Leslie has owned her own business, Imagine That, retailing traditional photography products, photographic restoration, custom printing and video conversions. She finds her Zen next door at her studio, the Mecca, where she plays with her film cameras, processes film and holds small classes focusing on teens and young adults.