Motion smear

Lifted from the decision tree in Break/Fix — unexpectedly soft images and wrong reciprocity corrections, whose first question separates movement from optics and settles half the cases on the page.

What you see

A specific direction is soft and the perpendicular direction is not. An edge running one way is sharp; an edge running the other way is smeared.

Under a loupe, the ramp across a smeared edge is widest for an edge perpendicular to the motion and narrows to nothing for an edge parallel to it. That direction-dependence is the whole signature.

Three sub-cases, distinguished by what is smeared.

• Everything in the frame smeared the same way and by the same angular amount — the camera moved.
• Only some objects smeared while the static ones stay sharp — the subject moved.
• A faint complete second image displaced by a small amount — the shutter jarred the camera at one end of the exposure.

Likely causes

1. Wind on a tripod, or a knock, during an exposure that is measured in minutes. Wall’s Dictionary puts the ordinary pinhole exposure at twenty or thirty times a lens exposure, so the camera has that much longer to be disturbed.
2. Subject movement. Leaves, water, cloud, traffic and people all move within a pinhole exposure, and this is normal rather than faulty.
3. The shutter itself, operated by hand on the front of the camera.

What is happening: the chemistry and physics

Every optical cause of unsharpness in a pinhole camera comes from the aperture, and a hole has no preferred direction. The blur figure is the shape of the hole, so it is the same shape whichever way an edge in the subject happens to run.

Movement is different in kind. Displacement during the exposure smears the image along the direction of travel and leaves the perpendicular direction untouched, so an edge parallel to the motion is imaged onto itself and stays sharp.

The exception that proves the rule is a burr, which also has a direction - but a burred aperture still has width perpendicular to the burr, so it blurs perpendicular edges as well, and its smear is the same at every point in the frame rather than following what moved.

✦Hint 2 from the break/fix page, and it is worth memorising

Take one high-contrast edge and look at it under a loupe twice — once along the edge and once across it. If the two look different, you are finished. No optical cause on the tree can produce a direction-dependent blur.

Diagnostic questions

• Two perpendicular edges under a loupe. Different, and it is movement.
• Is everything smeared, or only some things? Everything means the camera; some things mean the subject.
• Is there a faint complete second image? That is a jar at one end of the exposure — or, if the second image is displaced and the sheet was loose, a sheet that changed shape during the exposure.
• Re-shoot. One frame of a static, high-contrast subject, on a tripod, in still air, at a computed exposure, with everything logged. Half the cases on the break/fix page are settled by whether the fault survives that.
• What does the log say moved? The log has a field for it for this reason.

Corrective action

Nothing on the negative. A smeared exposure has recorded a moving subject, which is a fact about the world rather than a defect in the camera.

Re-shoot with the camera weighted or shielded, with the shutter operated so that it does not load the camera, and with the subject chosen or timed for stillness.

Prevention

Weight the tripod, shield it from wind, and do not touch the camera during the exposure — operate a sliding shutter with a light touch at one end and count the time rather than watching the camera.

Log what moved. A negative whose log says “wind, gusting, trees in frame” is a record; one whose log says nothing is a puzzle.

Where movement is inevitable, decide about it in advance. A pinhole exposure of minutes makes moving subjects into a picture-making choice rather than a fault, and the art part of the course treats it as one.