Static discharge marks

Owned by the emulsion under stress.

What you see

Kodak’s Z-133 gives the description precisely enough to use as an identification, and it names all three shapes: “branch-like marks, circular spots with dark centers, row of spots (often surrounded by fogged areas)”.

They are dark on the negative, because they are exposure. They are sharp. They ignore the frame lines, crossing into the inter-frame gaps and the rebate, and they ignore the picture entirely.

The branching form is the one people recognise: a tree or a fern, with a trunk and finer and finer subdivisions.

Likely causes

1. Winding or rewinding quickly, especially in a dry room in winter.
2. Separating sheets or rolls fast — pulling interleaved sheet film apart, or peeling backing paper.
3. Low humidity in the space where the film is handled.
4. A bulk loader or a camera transport that drags the film over an insulating surface.

Kodak’s three corrective actions are the three variables: maintain moderate levels of humidity and temperature in the areas where film is handled, handle film carefully, and separate rolls and sheets slowly and carefully.

What is happening: the chemistry and physics

Film is an insulator — ILFORD’s published bases are acetate and polyester, 0.110 to 0.180 mm thick — moving over other insulators. Separating two insulating surfaces separates charge, and in dry air there is nothing to carry it away, so the potential builds.

When the potential is high enough, the air breaks down and the charge crosses it. A spark emits light. The film is inches away, sensitive, and in the dark, so it records the spark like any other exposure. It is literally a photograph of an electrical discharge, taken by the film that made it.

The shapes follow from the physics of the discharge rather than from anything photographic:

• A branching mark is a discharge propagating across a surface, subdividing as it goes — the same tree structure that a lightning flash has, and for the same reason.
• A row of spots is a series of discharges at intervals as the film moved past a fixed point.
• A spot with a dark centre and a fogged surround is a point discharge photographed from very close, with the general glow around it recorded as well as the spark.

!Static or a light leak? Three observations, and they agree

Both are unwanted exposure, so both are dark on the negative, and they are confused for that reason.

Sharpness. A static mark is sharp and structured — a branch, a dot, a row. A light leak is a gradient with a soft falloff, because light from a hole falls off with distance.

Geometry. A static mark ignores the camera entirely: it appears wherever the discharge happened, crossing frame lines and running into the rebate, in a place that has no relation to the camera back or the cassette mouth. A leak’s position is fixed by the hole that made it and repeats accordingly.

Season and situation. Static appears in winter, in heated rooms, on fast winds and rewinds, and vanishes when the air is damp. A leak does not care what month it is.

Three independent observations, and they are unlikely to disagree.

Diagnostic questions

• Is the mark branched, or a row of dots, or a spot with a dark centre? Any of the three is characteristic and nothing else in this atlas produces them.
• Is it sharp, or does it fade out? Sharp is a spark; graded is a leak.
• Does it cross frame lines and run into the rebate? Static does, freely.
• When was it, and what was the air like? Winter, central heating, and a fast rewind is the classic combination.
• How fast was the film wound or rewound? And was a motor drive used?
• Was it the last few frames, at the tight end of the roll? Cinching and fast winding both concentrate there.
• Is it one camera or one loader? Bulk loaders are a common source, because the film is dragged over felt.

Corrective action

None. The mark is a developed image of a spark, chemically identical to the picture, and no treatment distinguishes them.

Where it falls outside the picture area or in a dark region of the frame, the negative may be usable. In the sky, it is not.

Prevention

All three of Kodak’s corrective actions are preventions, and all three are free.

Wind and rewind slowly, and rewind by hand rather than with a motor where the choice exists.

Separate sheets and rolls slowly and carefully, which is Kodak’s own phrasing and is the whole answer for interleaved sheet film.

Keep the humidity moderate in the room where film is handled. This is the variable people ignore and it is the one that decides whether the charge can leak away at all; a heated room in midwinter is the worst case and a bowl of water near the bench is a real if unglamorous measure.

Do not drag film over insulating surfaces — a plastic bench, a nylon changing bag being pulled tight.

An anti-static treatment exists for dry film in handling, and ILFORD list it among the uses of their wetting agent, but note the scope: that is about processed negatives being handled and cleaned, not about film in the camera, and it does nothing for a discharge that happens before development.