Seam leak

Lifted from commissioning the pinhole camera, whose expected-observations section reads the shape of a leak as its name.

What you see

A sharp streak or wedge, usually running in from one corner and widening as it goes, with everything else on the sheet clean. Like every leak, it continues under the strips the paper stops covered.

The hard edge is the point. It is the difference between this and an edge leak, and it is telling you something specific about the path the light took.

Likely causes

1. An unfilled screw hole.
2. A crack or split in a panel.
3. A gap at a rebate, where two panels meet edge to edge with no overlap to interrupt a straight line.

What is happening: the chemistry and physics

A small, well-defined aperture in the body is a pinhole, whether you meant it to be one or not. Light entering through it travels in straight lines to whatever part of the sheet is in its line of sight, and the boundary of the illuminated region is the shadow the hole’s own geometry casts. That is why the mark has a hard edge and a definite shape.

A seam that runs along a whole edge behaves differently, because it admits light along its length and at a range of angles, so what arrives is spread and soft. Hard means a point source; soft means a line source. That single physical distinction is the whole of the discrimination between the two entries.

The wedge shape follows from the same reasoning — the region a small hole can illuminate widens with distance from it.

Diagnostic questions

• Hard edge or soft gradient? Hard is this entry, soft is the edge leak. Read it on the leak sheet rather than on a picture, where subject detail confuses the boundary.
• Does it cross the paper-stop strips? It must, or it is not a leak.
• Torch test. In a dark room, put a bright torch inside the closed camera and look at it from every angle. A straight path shows itself immediately, and so does the screw hole nobody remembered.
• Which of the two commissioning sheets carries it? Closed shutter, full sun, thirty minutes tests every path except the shutter; capped pinhole with the shutter open tests only the shutter. A mark on the first is a body path.
• Trace the wedge back. The apex points at the hole.

Corrective action

Fill the path rather than covering the outside of it. A screw hole is filled; a crack is backed from inside; a rebate is given an overlap or a light trap.

Tape on the outside is a valid diagnostic — it proves you have found the right path — and a poor repair, because tape at a joint that flexes will fail exactly when the camera is in the sun.

Re-run the closed-shutter sheet alone after the repair, and record which pass found it.

Prevention

Design joints with an overlap rather than a butt, so that no straight line exists from outside to the film plane. That is a build decision and it is much cheaper than a repair.

Paint or flock the interior matt black, which does not stop a leak but stops the light that does get in from being scattered efficiently onto the sheet.

✦Two leaks look like one

If a camera fails the closed-shutter test, is taped, and fails again, you have two paths rather than one stubborn path. Test after each repair and record it, or the second leak will be attributed to the first repair having failed.