Age fog

Owned by fog and where it comes from.

What you see

Muddy shadows before anything else. A film or paper past its useful life produces images whose low tones will not separate: the shadows are grey rather than dark, the print looks flat regardless of grade, and the highlights still look more or less correct.

On the film itself, the rebate and the unexposed leader are darker than they are on a fresh roll of the same stock. On paper, the base will not stay white.

It is even — over the whole sheet, over the whole roll, including the rebate — because it is a property of the material rather than of anything that happened to it in your darkroom.

Likely causes

1. Time alone. Every emulsion acquires developable grains as it sits.
2. Warm storage, which accelerates it. Kodak ask for unexposed film at 24 °C or lower in the original sealed package; ILFORD ask for 10–20 °C, dry, in the original packaging.
3. High humidity. Both makers name it alongside temperature.
4. A fast emulsion. The properties that make a film sensitive to light — larger grains, more silver, more surface chemistry — also make it likelier to produce a centre without light.
5. A long interval between exposure and development, which is a related but distinct fault: there the loss is of latent image rather than a gain of fog, and both makers ask for film to be processed “as soon as possible” or “as soon as practical” after exposure.

What is happening: the chemistry and physics

Development amplifies a sensitivity centre: a small speck of silver on or in a grain that catches the developer and lets the whole grain go. Light is the ordinary way to make one, but it is not the only way. Given time — and much less time at higher temperature — the same centres form without any exposure, and a grain that has one is developable whether or not a photon ever reached it.

Fog therefore arrives before speed leaves, which is the practically useful asymmetry. The first grains to acquire centres by ageing are the ones that were already the most sensitive, and they are relatively few, so the effect at first is a raised floor rather than a visibly slower film. That is why an old film’s first symptom is a muddy negative rather than a thin one.

The floor eats the shadows first, because near the toe the curve is nearly flat: a raised floor swallows the small density differences that were the shadow detail, while the highlights, on the straight line, barely notice.

✦One test tells you what a box of old film is worth

Cut a short length in the dark, develop it with no exposure at all, fix and dry it, and lay it beside a strip of fresh stock treated identically.

The difference in density is the fog the film has accumulated, measured on your own material with your own developer — which is better information than any expiry date, because the date says nothing about how the box was kept. Old film that was kept cold can be excellent; the same film off a warm shelf can be useless.

Diagnostic questions

• Is the rebate raised against a known-good strip of the same stock? If not, this is not age fog.
• What does an unexposed, developed strip of the same box read? The measurement above.
• How was it stored, and for how long? Not the printed date — the actual shelf.
• Is it a fast film or a slow one? A 400 or 3200 film from the same era as a 100 film will usually be worse.
• Does the emulsion also seem slower? Speed loss arriving after the fog is consistent with age; speed loss without fog is a different problem.
• Is it every roll from the batch, or one? A batch points at storage; a single roll points at that roll’s own history.

Corrective action

Nothing on the material already exposed. Fog is developed silver and it is in the picture.

For the rest of the box, three real options. Rate the film generously — a stop more exposure puts the shadows further above the raised floor, at the cost of more density everywhere. Cut the development, which lowers the fog as well as the contrast. Or add a restrainer: Kodak Limited sold antifog tablets for exactly this case, “when materials have deteriorated through age or incorrect storage conditions”, and Part VIII owns the chemistry and the concentrations the course teaches.

And the honest option: for work that matters, buy fresh film. Old stock is for testing and for pictures that can be repeated.

Prevention

Store as the maker asks: cool, dry, in the original packaging — ILFORD’s 10–20 °C or Kodak’s 24 °C or lower. Both permit a fridge or freezer with time allowed to acclimatise before opening, which matters because condensation on a cold film is its own fault.

Buy what you will use. Fog accrues on the shelf whatever the shelf belongs to.

Process exposed film promptly, which is a separate instruction from both makers and prevents a separate fault.

Record your own base plus fog for the stocks you use, from a good session, so that a departure is a number rather than an impression. No datasheet in this course’s corpus publishes the value, so yours is the only reference available.