Burr smear

Lifted from Break/Fix — unexpectedly soft images and wrong reciprocity corrections, case 1.

What you see

The whole frame is soft, and the softness has a preferred direction that is the same everywhere in the frame. Every point is smeared the same way, by the same amount, whether it is at the centre or in a corner.

Under a loupe the density profile across a hard edge is asymmetric — it spreads further on one side of the edge than the other — and it spreads the same way at every point in the frame.

Likely causes

1. Piercing thin metal pushes a lip of material up on the far side. That lip stands into the light path.
2. A hole finished from one side only, so the burr was never abraded away.
3. A plate reworked after it was measured, leaving both a burr and a diameter that is no longer what the register says.

What is happening: the chemistry and physics

The blur figure of an object point in a pinhole camera is the shape of the aperture. A clean round hole gives a round blur; a hole with a flap standing proud on one side gives a lopsided blur.

And every point in the frame sees the same hole, so every point gets the same lopsided blur. That is the whole signature, and it is what separates a burr from movement.

Wall’s 1912 Dictionary gives the instruction without the explanation — “the edges of the pinhole should be quite clean and free from burr” — and it has been true for as long as people have pierced holes in metal.

Diagnostic questions

• Why it is not movement. Motion smear also has a direction, which is why the two get confused. The difference is that motion leaves an edge running parallel to the motion completely sharp, and a burr blurs perpendicular edges as well, because the aperture still has width in that direction. Look at one high-contrast edge under a loupe twice — once along it and once across it.
• The laser diffraction pattern from the pinhole lab. A clean circular hole gives concentric rings; a burred hole gives a fan, a streak, or rings that are brighter on one side. This is the confirming test.
• The loupe on the plate itself, from the burr side, and a scan at 4800 dpi, are the second and third checks.
• Is the softness the same amount in the corners as at the centre? Aperture faults are uniform across the frame; geometry faults are not.

Corrective action

Re-make the plate. That is the honest first answer for a plate whose diameter matters.

A burr can be abraded off with fine wet-and-dry on a hard flat backing, working from the burr side - and the hole must then be re-measured, because abrading changes the diameter. If the hole was already at the small end of its band, abrading will take it out of the band entirely and you will have traded a burr for diffraction glow.

Prevention

Pierce from one side only, part way, then reverse the plate and abrade.

Measure after every operation, not just at the end. The diameter is not a property of the intention; it is a property of the metal after the last thing you did to it.

Record the result in the plate register, so that a plate which has been reworked is identifiable later, and note the optimum diameter for the focal distances the plate will be used at beside it.