Light-leak fog

Owned by Fog and Where It Comes From, which reads the four things a leak’s signature tells you: it has a direction, because light falls off with distance; it usually has an edge, which is a picture of whatever cast it; it often repeats with the geometry of the material; and whether it stops at the frame line says whether it happened in the camera. This entry was written before that page from commissioning the pinhole camera, whose six-signature comparison does the same discrimination for a home-made camera.

What you see

Density that the picture cannot account for, in a place the picture could not have reached. The shape is what names the path.

• A broad soft band along one edge, densest at the edge and fading inwards, is an edge leak — a seam, a corner, or the mouth of the dark-slide slot.
• A sharp streak or wedge is a straight path and is a seam or hole leak — a screw hole, a crack, or a gap at a rebate.
• A band along the edge nearest the shutter, present on one commissioning sheet and not the other, is a shutter leak.
• A general grey with clean white borders where the paper stops covered the sheet is not a leak at all. It is fog reaching the whole emulsion, which means it happened before or after the camera — the box, the safelight, or handling.

This entry is the family; the four rows above are its members, and the value of having the family is that the two questions below dispose of it in a minute.

Likely causes

1. A path into the camera body.
2. The shutter’s own light trap.
3. A storage box, changing bag or dark slide that is not light-tight.
4. A loading routine carried out somewhere light can reach the material.

What is happening: the chemistry and physics

Light that did not pass through the hole forms no image, so it deposits exposure wherever it can land, and the geometry of the entry point decides the shape. A point-like opening throws a hard-edged patch; a long seam throws a soft gradient; light that reached the sheet outside the camera reaches all of it.

The reason a leak too faint to see matters is the exposure time. A pinhole exposure is minutes, and the paper reports the leak multiplied by all of them. A leak far too faint to see can put half a stop of density along one edge of a sheet over thirty minutes.

Paper punishes leaks more than film does, because it is a short-scale, high-contrast material. The same leak on a long-scale film would put down a density difference you would need an instrument to find.

!The two questions that do almost all of it

Does the mark continue under the strips the paper stops covered? Light that came through the hole - the picture, flare, a hot spot — stops dead at the edge of the image circle. Light that came in any other way does not.

Is it on the unexposed control sheet? The control carries the box, the safelight and the developer and nothing else. Clean control, veiled picture, and the fault is inside the camera.

Diagnostic questions

• Run the two commissioning sheets. Shutter closed, thirty minutes in full sun tests every path except the shutter. Shutter open, pinhole capped, thirty minutes in full sun tests the shutter and nothing else.
• Read the results as a pair. Sheet 2 fogged and sheet 3 clean is a body path. Sheet 3 fogged and sheet 2 clean is the shutter alone. Both fogged with clean paper-stop strips means light came through the pinhole and the cap was not properly on.
• Both leak sheets clean and all the picture sheets grey in the same place points downstream of the camera — look at the dark slide, the box and the loading routine, in that order.
• A torch inside the camera in a dark room finds most paths in a minute.

Corrective action

Find the path, name the edge, seal it, and re-run the one sheet that found it. Testing one repair at a time is what stops a camera with two leaks from being reported as a camera with one stubborn leak.

Tape is a diagnostic. A permanent repair fills the hole, backs the crack or cuts the rebate.

Prevention

Repeat the leak test after any work on the camera and after adding a frame to the stack — each frame adds two joints, and a stack that passes at one focal distance can leak at another.

Run the test at thirty minutes as standard, and at sixty and a hundred and twenty when you want a number for the faintest leak the camera has rather than a pass or a fail.

Record every leak found and sealed, with its date and the pass that found it, in the camera’s own leak table.