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This artefact indicates unintentional exposure from the backside of the film.
(As the red-sensitive layer, which is the lowest, next to the film-base, is the same time the most sensitive one. The red couplers and a red filter on top of this layer shield the layers above.)
Look for a light-leak in the back of your camera. Or at the side or front if light from there could reach just the film on the take-up spool, if it is wound base-side out.
Last edited by AgX; 07-12-2013 at 11:26 PM. Click to view previous post history.