Geometric softness from an oversized hole

Lifted from the decision tree in Break/Fix — unexpectedly soft images and wrong reciprocity corrections, whose left-hand optical branch separates it from its two neighbours, with the optimum from diffraction and the optimum pinhole.

What you see

The whole frame is soft, evenly, the same in every direction and by the same amount at the centre as in the corners.

Under a loupe, the density profile across a hard edge is a straight ramp of definite width with sharp corners where it meets the black and the white. The unsharpness has a limit you can point at — it is plain blur rather than glow.

Likely causes

1. A hole larger than the optimum for the focal distance being used.
2. The right plate for a short focal distance used at a long one, or the wrong plate taken out of the register.
3. A hole that was pierced further than intended and not re-measured afterwards.

What is happening: the chemistry and physics

A pinhole has no focus to lose. A point in the subject at a great distance projects, through a hole of diameter d, a disc on the film of diameter d — and a uniformly lit round hole projects uniform illumination out to a hard limit, which is why the edge profile is a flat-topped ramp with corners rather than a peak with tails.

That geometric term rises in proportion to the hole diameter. The diffraction term falls in inverse proportion to it. The optimum hole is where the sum of the two is least, which is what Petzval minimised in 1857 and what Rayleigh’s quarter-wave criterion arrives at from the wave side; above the optimum the geometric term dominates and this entry is what you get.

The blur circle is the same size everywhere in the frame, because every point in the frame sees the same hole from the same distance. Uniformity across the frame is therefore part of the diagnosis, not an incidental.

Diagnostic questions

• Look at the log before the negative, and specifically at the plate letter and the focal distance together. Most softness on a first camera is a hole that is wrong for the distance it is being used at, which is arithmetic rather than diagnosis.
• Two perpendicular edges under a loupe. If they differ, it is movement and no optical cause can produce it, because a hole has no preferred direction.
• Ramp or glow? A straight ramp with hard corners is an oversized hole. A narrow core with far-reaching tails is diffraction from an undersized one, and it reads as a glow spilling across the edge rather than as a loss of definition.
• Asymmetric? Spreading further on one side, the same way at every point, is a burr.
• Is the whole frame affected? Part-frame softness lives in the geometry - the near limit, plate thickness, a sheet that was not flat.

Corrective action

Fit the right plate for the focal distance, or change the focal distance to suit the plate you have. Nothing else recovers it, because the information was never recorded.

Re-measure the plate you were using, by a second method, before you put it back in the register.

Prevention

Compute the optimum diameter for every focal distance the camera offers, and mark each plate with the distances it belongs to rather than with its diameter alone. A register that says “0.30 mm” tells you what you made; a register that says “0.30 mm, optimum near f = 120 mm” tells you what to do with it.

Run the diameter series from the commissioning session — three plates at one focal distance, with the times scaled by the square of the f-number so that only sharpness changes — and keep the sheets. You then have your own comparison rather than a prediction.