Loading 120 without ruining the roll
GuidesThe paper backing is the whole trick.
Backing paper, not a cassette
120 film has no light-tight cassette. The paper backing is the only thing protecting the emulsion, so how tightly you keep the roll wound matters more than anything else you do.
The sequence
Seat the fresh roll, thread the leader across to the take-up spool, and wind until the arrow or index mark lines up with the camera's mark. Keep light pressure on the roll the whole time — a loose roll admits light along the edges.
After the last frame
Wind fully onto the take-up spool, then fold the gummed tab under and press it down before the roll leaves your hand. A half-wound roll set down on a table is how edge fog happens.
Storage
Exposed rolls travel emulsion-in, in something opaque. Heat is a slower problem than light but it is still a problem.