Hot spot

Lifted from Break/Fix — unexpectedly soft images and wrong reciprocity corrections, case 5, and from the six-signature comparison in commissioning the pinhole camera.

!A prediction, not an observation

This course has not photographed a hot spot. The signature below is derived from the geometry of a specular reflection inside a box, and it is offered as a prediction to be tested rather than as something somebody looked at. If your own camera produces one, your photograph is better evidence than this page, and the entry is written so that it drops straight in.

What you see

A broad bright patch on the negative near one edge — which is to say a light area on a paper negative, since a negative is inverted and extra exposure makes density. It is roughly the shape of the interior wall it came from, and it sits on the opposite side of the frame from the offending source.

Like flare, it stops at the strips the paper stops covered, because the light came through the hole.

Likely causes

1. A flat interior wall that reflects specularly — bare metal, foil, gloss paint, smooth plastic.
2. A source just outside the frame, striking that wall at a shallow angle. Grazing incidence is where many nominally matt finishes behave as mirrors.
3. An interior that was blackened but not flocked, so that it absorbs at normal incidence and reflects at grazing incidence.

What is happening: the chemistry and physics

Flare and a hot spot are the same light behaving in two different ways. Light that misses the film lands on a wall. A diffuse reflector scatters it in all directions and what returns to the sheet is a more or less even veil. A specular reflector returns it in one direction, so what returns is an image of the wall rather than a wash over the frame.

The geometry decides the side. A reflection off the wall on the source’s own side is thrown across the camera to the far side of the sheet, which is why the patch appears opposite the source rather than beneath it.

The patch is soft-edged because the wall is an extended source and the hole is not re-imaging it — there is no optics between the wall and the film, only distance.

Diagnostic questions

• Does the patch stop at the paper-stop strips? It must. If it carries on under them, it is a leak and not a reflection.
• Where was the sun? A hot spot is on the far side of the frame from the source. If the bright patch is on the same side as the source, look for a leak.
• The torch test, done properly. A torch inside the camera in a dark room, moved to the angles the sun actually occupied, looking for anything that shines at grazing incidence. A wall that looks matt from in front may not be.
• The flare pair. The flagged sheet should lose the patch as well as lifting the shadows less.
• Is the patch roughly the shape of a wall? That is the clue in the name.

Corrective action

Matt black paint on the offending wall, then flocking or black felt over it. Flocking works at grazing incidence, where paint often does not, which is the whole reason it is worth the trouble.

Flag the source when it is near the edge of the field.

Prevention

Blacken and flock the two walls that face the film most directly when the camera is built, rather than after a session has been lost.

Blacken the pinhole plate on both faces. An unblackened plate is a small mirror parallel to the film at the focal distance, and it produces a different mark - a faint displaced ghost rather than a patch.

Photograph a bright, evenly lit blank wall filling the frame, with the sun at the angle that troubles you. The result is a map of the camera’s own illumination with no subject structure in it, and any hot spot is then unmistakable.