Diffraction glow from an undersized hole

Lifted from the decision tree in Break/Fix — unexpectedly soft images and wrong reciprocity corrections, with the physics from diffraction and the optimum pinhole.

What you see

The whole frame is soft, the same in every direction, and soft in a particular way — with a faint glow around every edge rather than a plain even blur.

Under a loupe, the density profile across a hard edge has a narrow steep core with tails that reach much further into both the black and the white than the core is wide. The profile is peaked rather than flat-topped, and the eye reads it as light spilling across the boundary.

The commissioning session predicts this for the smallest plate in the diameter series, and it is the one prediction on that page that a beginner can usually see.

Likely causes

1. A hole smaller than the optimum for the focal distance in use.
2. A hole taken below its band by abrading a burr off it, without re-measuring.
3. A plate made for a long focal distance and used at a short one.

What is happening: the chemistry and physics

Light passing an aperture spreads, and the smaller the aperture the more it spreads. For a circular hole the first minimum of the pattern sits at an angle of 1.22 λ / D, so the angular spread grows as the hole shrinks — which puts it in direct opposition to the geometric blur, which shrinks as the hole shrinks.

The two together give the optimum. Rayleigh’s quarter-wave criterion produces the relation between hole size and focal distance that this course uses, and he backed it with a photographic determination rather than only with theory.

The shape of the blur is what identifies it. A geometric blur is the projection of a uniformly lit disc, so it is flat-topped with hard limits. A diffraction blur is a bright central core surrounded by faint rings, so it has a peak and a skirt. Two negatives with the same nominal blur width can therefore look entirely different, and the difference is the diagnosis.

📷Why it shows on paper more than you expect

Photographic paper is blue-sensitive, and the diffraction spread is proportional to wavelength. A material that responds mainly to short wavelengths therefore diffracts less than a panchromatic film in the same camera — so a paper negative is, on this one axis, the more forgiving material. What paper is not forgiving about is contrast, which is why the glow is easy to see on it once it is there.

Diagnostic questions

• Glow or ramp? A peaked core with long tails is diffraction. A straight ramp with hard corners is an oversized hole. This is the confirming comparison, and it is made with a loupe on one high-contrast edge.
• The log, first. A soft negative made with the smallest plate at the longest focal distance is diagnosed before the sleeve is opened.
• Is the softness the same in every direction? It must be, or the cause is movement.
• Was the plate abraded recently? Abrading a burr off reduces the diameter, and a plate at the small end of its band goes out of it.

Corrective action

Fit a larger plate, at the optimum for the focal distance in use.

Do not try to compensate with exposure. The glow is not an exposure effect and no amount of extra time sharpens it — and the small hole has already cost you the time, because the f-number rose with the square of the diameter reduction.

Prevention

Compute the optimum diameter for every focal distance the camera offers, and keep the plate register with the diameters, the thicknesses and the distances each plate belongs to.

Re-measure any plate you have worked on. A plate that has been abraded is a different plate.

Run the diameter series and keep the negatives — three sheets at one focal distance, times scaled by the square of the f-number so that only sharpness changes. Expect the differences between the two larger holes to be small; the prediction the course makes for its own example is 18 per cent, 2 per cent and 1 per cent, and only the first is near the limit of what a loupe resolves.