Double image from a reflective plate

Lifted from Break/Fix — unexpectedly soft images and wrong reciprocity corrections, case 5, where it is set out beside the hot spot as the second of two specular faults.

!A prediction, not an observation

This course has not photographed this ghost. It is described from the geometry of a small mirror parallel to the film at distance f, and it is offered as a prediction to be tested rather than as something somebody looked at. Treat the description as a hypothesis with a shape, and if your camera produces one, your negative is the better evidence.

What you see

A faint, broad, displaced ghost of the frame’s own bright areas — not a sharp second picture.

It sits over an otherwise normal negative, it has no hard edges, and it corresponds to the parts of the frame that received the most light rather than to the whole picture.

Like every fault whose light came through the hole, it stops at the strips the paper stops covered.

Likely causes

1. A pinhole plate blackened on the outside only, so that its inside face is bare metal.
2. A plate used as made, with no blackening.
3. Blackening abraded off the inside face by handling or by repeated fitting.

What is happening: the chemistry and physics

Light reaches the film, and some of it scatters off the emulsion and the support rather than being absorbed. That scattered light travels back towards the front of the camera.

An unblackened pinhole plate is a small mirror parallel to the film at distance f. What it returns is light that has already been to the film once, spread by the scattering that sent it back, so what arrives is broad, faint and displaced rather than a second sharp image. The bright areas of the frame sent the most light forward, so they are what the ghost is made of.

This is why the mark differs from a hot spot, which is a reflection of a wall and takes that wall’s shape, and from flare, which is diffuse and even across the frame.

Diagnostic questions

• Does the ghost stop at the paper-stop strips? It must, or the light did not come through the hole and you are looking at a leak.
• Is the ghost made of the frame’s own bright areas, displaced? That is the discriminator. A hot spot is the shape of a wall; a leak is the shape of a gap; this is the shape of the picture.
• Look at the inside face of the plate. Bare metal, or blackened? This is the test that costs nothing and answers it, and it is the reason the entry is worth having even without a photograph.
• Torch inside the camera, in the dark. The plate should not shine from the film’s point of view.
• Two sheets, one with the plate blackened on both faces. A controlled comparison, and the only evidence that would turn this prediction into an observation.

Corrective action

Blacken the inside face of the plate. Matt black paint, or a flocked ring around the hole that does not encroach on the aperture.

Re-measure the hole afterwards if any material has gone near it, because paint in the aperture changes the diameter and the geometry.

Prevention

Blacken every plate on both faces as part of making it, and record in the plate register that you did. This is a one-line habit that removes the fault permanently.

Handle plates by their edges and store them so that the blackening is not abraded off in a box.

If you make the controlled two-sheet comparison, keep both negatives and write the result into this entry’s place in your own atlas. The course would rather have your evidence than its geometry.