Originally Posted by
Ken Lee
If the streak appears on the tray-developed negative, you may have a spoiled or scratched batch of film - or you're scratching them during handling, loading, etc. If not, you've ruled out those factors.
To that point, it's helpful to clean your fingers with rubbing alcohol (and allow them to dry) just before handling sheet film. That way you can't accidentally mark them with natural oils that would inhibit the contact of all chemicals with your film. If you've mixed any chemicals prior to handling the film, alcohol will help remove them. I arrived at this method after getting finger-prints in my negatives which appeared white in the final print: hard to remove even with Photoshop.
The next step would be to expose 4 sheets (or however many your tank can accommodate) to dim light and develop them all in your tank simultaneously. If the streak appears on one or more negatives, there's something in one or more of the slots in the tank (or the way you're loading them) which is limiting developer contact with the film.
To identify each sheet, you can cut from 1 to 4 notches into each sheet: be sure to record which sheet went into which slot of the tank.
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