Shutter leak

Lifted from Break/Fix — unexpectedly soft images and wrong reciprocity corrections, case 6, whose confirming test was built in commissioning the pinhole camera.

What you see

A band of raised density along the edge of the sheet nearest the shutter, on a sheet whose picture is otherwise normal. Like every leak, the band crosses the strip that was under the paper stop.

Likely causes

1. A sliding blade that lies flat against the outside of the front panel rather than running in a rebated channel.
2. A gap that opens at a shallow angle between the blade and the panel.
3. The simplest cause of all, and worth ruling out first — the shutter was not properly closed.

What is happening: the chemistry and physics

A blade lying flat on a surface makes contact along a line, not over an area. At shallow incidence, light finds a wedge-shaped gap between the two, and once it is inside the camera it has no image to form because it did not pass through the hole.

So it illuminates whatever part of the sheet it can reach, and the part it reaches is the part nearest the gap. The gradient falls away from the shutter edge.

The gap points inwards, which has a practical consequence that catches people out — tape on the outside of a blade lying flat will not fix it, because the leak is on the inside face of the joint.

Diagnostic questions

This is the fault with the cleanest confirming test in the whole camera, because the commissioning session built two sheets specifically to separate it.

• Sheet 2 — shutter closed, thirty minutes in full sun. Tests every path except the shutter.
• Sheet 3 — shutter open, an opaque cap taped over the pinhole, thirty minutes in full sun. Tests the shutter and nothing else, because the shutter is the one path sheet 2 keeps shut.

Sheet 3 dark and sheet 2 clean is unambiguous — the shutter’s light trap is the whole fault, and it cannot be anything else. Both sheets dark points at the body instead. Both clean with a fogged picture sheet points downstream of the camera, at the dark slide, the box or the loading routine.

One more check is worth having. If both leak sheets are fogged and the edges under the paper stops are clean, the light arrived through the pinhole, which means the cap or the shutter was not actually closed. Repeat with the cap taped on all four sides.

Corrective action

Cut the rebated channel, so that the blade runs in a groove and the light path is interrupted by a step rather than by contact pressure.

Where that is not possible, a felt or flocked light trap inside the joint does the same work, and it is inside because that is where the gap is.

Re-run sheet 3 alone after the repair.

Prevention

Re-run both leak sheets after any work on the front of the camera, and after adding a frame to the stack - a stack that passes at 50 mm can leak at 120, because each frame adds two joints.

Check the shutter by eye in daylight before every session, closed, from the front. It costs five seconds and it eliminates the cause that is not a fault at all.

Record the leak test in the leak table with its duration. A camera whose shutter passed at thirty minutes has not been shown to pass at ninety.