Buckled-sheet softness

Lifted from Break/Fix — unexpectedly soft images and wrong reciprocity corrections, case 4, which works the consequences through rather than assuming them.

What you see

A denser centre, straight lines that are subtly bowed, and — here is the trap — a centre that is very slightly sharper rather than softer.

It is usually reported as softness because the eye reads the mismatch between a bowed line and a straight one as a defect of definition, and because a denser middle looks wrong without looking like anything in particular.

Likely causes

1. The sheet is not held flat in the back.
2. A fresh packet of resin-coated paper, which curls more than paper that has been in a camera all week.
3. Foam on the pressure plate that has compressed, or a single central spring on a long side.

What is happening: the chemistry and physics

Everything about a pinhole scales with the focal distance f, so a bow that changes f locally changes three things at once.

Take a sheet that bows so the centre is 2 mm closer to the hole at f = 50 mm — a 4 per cent change.

• Illuminance goes as 1/f², so the centre receives about 8 per cent more light. A denser middle.
• Magnification is f/u, so the middle of the picture is drawn at a slightly smaller scale than the edges, which bows straight lines.
• The blur circle is proportional to f, so the centre’s blur is 4 per cent smaller. The middle is fractionally sharper.

So a buckled sheet is a density fault masquerading as a sharpness fault, and diagnosing it as softness sends you to inspect the plate, which is fine and will be found innocent.

Diagnostic questions

• A straight-edge across the loaded back before the dark slide goes in. This is the direct test and it takes a moment.
• Is the density gradient radial about the middle of the paper, or about the optical axis? A cos-fourth gradient is symmetric about the axis; a bow gradient is symmetric about the centre of the sheet — and if your carrier is offset, those are not the same point. This is the confirming test on the negative itself.
• Are straight lines in the subject subtly bowed?
• Is the centre sharper than the edges? Almost nothing else in the camera does that.

Corrective action

Thicker foam on the pressure plate, or a second leaf spring at the middle of the long side.

The negative already made cannot be corrected, but it can be labelled correctly, which matters - recorded as a flatness fault it stops the plate from being blamed.

!The related movement fault

A sheet that is not properly held can change shape during the exposure, popping from one curvature to another. That gives a faint complete second image, displaced by a small amount, over an otherwise normal negative.

It is a movement fault rather than an optical one, so it belongs on the other branch of the tree entirely — and it is not the same as the ghost from a reflective pinhole plate, which is a reflection and displaces the bright areas rather than the whole frame.

Prevention

Load one sheet, close the back, and check with a straight-edge through the dark-slide slot before every session that starts a new box.

Let a fresh packet of resin-coated paper relax before it is used for anything that matters, and store cut sheets flat rather than in the curl the packet gave them.

Record the paper lot and the packet’s age in the exposure log, so that a run of bowed sheets can be attributed rather than guessed at.