Uneven contact print from pressure, glass or source geometry

Lifted from the contact printing frame and its light source, whose stage 1 derives the penumbra arithmetic and whose T1, T2 and T3 are the tests below; from Break/Fix — the room that fogs and the light that is not even, which owns the unevenness differential and the paper-rotation test; and from ultraviolet sources and dose, which owns the evenness of an array.

What you see

Two unrelated faults share one complaint. A contact print that “came out uneven” is either uneven in tone — density varies while everything stays equally sharp — or uneven in sharpness — one tone throughout, but some regions crisp and others soft. The first is the light, the second the sandwich, and they share no cause, no test and no fix.

Use a loupe before judging it at arm’s length, where the two look alike: a smeared edge reads as low local contrast, and low local contrast reads as flat and grey. Under a loupe one has fuzzy edges, the other sharp edges at the wrong density.

What the sheet shows	Fault	What it points at
Smooth darkening at all four corners	Tone	Box too shallow, or too little overhang past the printing area
A regular stripe parallel to the emitter rows	Tone	Emitter pitch printing through a diffuser that is too close
A gradient from one side to the other, or one bright patch	Tone	Diffuser off its ledges, a strip fed from one end, or gloss inside the box
A soft, tent-shaped region tens of millimetres across	Sharpness	One grain of grit, the region centred on the speck
Ends of each 35 mm strip soft, middles crisp	Sharpness	Curl, lifting the ends of a strip off the paper
Middle soft and edges crisp, or the reverse	Sharpness	Glass or back bowing between the springs; or a weak spring, unseated latch or set foam
Every frame evenly soft and mirrored	Sharpness	Negatives in the wrong way up, printing through the base
Concentric coloured or grey fringes	Glass	Newton’s rings
A fine grain-like texture over everything	Glass	Anti-Newton glass, its texture printing at contact scale

On an alternative-process sheet the tone half shows as patchy maximum density, firm blue in the middle and thin at the corners; the sharpness half is usually worse than on silver, these processes using larger and closer sources.

Likely causes

The sandwich — everything that puts a gap between the two emulsions: too few springs or an unseated latch, a cupped baseboard, open-cell foam compressed where the springs press, curl, grit, a back thin enough to bow, and the negative laid base-down.

The glass — too thin for the area and bowing between the springs, Ware being explicit that 2 mm picture glass may bend or crack under pressure while 4 mm plate is the answer for a clipped frame; a salvaged sheet that is drawn rather than float, with a slow wave in it; Newton’s rings; an anti-Newton texture, which is a fix for the rings carrying a defect of its own; and acrylic glazing under a 365 nm array, a transmission fault so routinely misread as a geometry one that it belongs here.

The source geometry — a diffuser too close for the emitter pitch, too little overhang, a diffuser not seated, and a long strip fed from one end so the far end runs at lower voltage.

Not the printer at all — uneven development in the tray, which the paper-rotation test excludes, and session exposure drift, a change between sheets rather than across one.

What is happening: the chemistry and physics

A gap costs sharpness and does not cost density. The source is not a point: it has a width, and from the negative it subtends an angle that spreads every shadow edge, across any gap, into a penumbra.

b = S × g ÷ D

Geometric penumbra across a gap

b is the blurred band on the paper, S the source width seen from the negative, g the gap, D the source-to-negative distance. Ware writes it more usefully — a source’s aspect value is its largest linear dimension divided by its distance from the print, and blur is gap times aspect value — and that form is what condemns this particular geometry. He gives an aspect value of 0.01 for the sun and 4 for a light bed or the open sky, and a box whose diffuser is the size of the printing area sits in that row — aspect value near 1, and up to 4 as the box gets shallower. At gaps of 0.1 to 0.5 mm Ware’s light bed gives 0.4 to 2.0 mm of blur, against 0.001 to 0.005 mm in sunlight. Ware takes 0.3 mm as the onset of a fuzzy image, from the eye resolving about 0.1 mm at its near point. So this printer crosses his threshold at a gap of a third of a millimetre — which you cannot see, cannot feel through a closed frame, and will not discover until the sheet is dry. ILFORD publish FP4 Plus as 0.110 mm acetate in roll film and 0.180 mm polyester in sheets: a negative laid base-to-paper under a diffuser of aspect value near 1 blurs by that thickness in millimetres.

Why the gap leaves the tone alone. Over a large clear area every ray that would have arrived still arrives; the gap only redistributes light within about b either side of an edge, so exposure integrated over an area much larger than b is unchanged.

Why an uneven field leaves the sharpness alone. A corner is further from most emitters and sees them more obliquely, which costs it light — the cosine-fourth law is the extreme case of that geometry for a point source. But the source’s angular size barely changes across an 8 × 10 in printing area, so S ÷ D, and the blur, are much the same everywhere.

!The confirming test: take the negative out

The printer’s own T3 uniformity map, run as a diagnostic rather than an acceptance test. It changes exactly one thing: it removes every edge from the light path. Close the empty frame on its glass with no negative, expose a sheet to a mid-grey, process and dry it with the session, and read fifteen positions against the centre.

• The pattern survives. It is the light, and its shape names the fault against the first three rows of the table above.
• The pattern is gone. It was the sandwich or the glass. A blank field has no edge for a penumbra to smear, so a gap fault cannot appear on it — a proof, not an inference from absence.

The acceptance figure is a worst point 0.04 in density from the centre — borrowed from ILFORD’s safelight sheet, where it is the change meaning safelight conditions are inadequate. Nothing in this course’s corpus sets a uniformity tolerance for a contact printer, so the only published statement of what is detectable on paper stands in for one.

Then rotate the frame through 180° on the box, negative and paper untouched inside it. A pattern that stays put in the room belongs to the box; one that travels with the frame belongs to the frame, its glass or the loading. No published account of this test was found in this course’s corpus; it is offered as reasoning, being the standard way of separating effects fixed to one component from effects fixed to another. Rotating the paper instead separates the tray, and the Break/Fix page owns that test and its limitation. The purely mechanical check is the frame’s T1, a 0.05 mm paper strip slid in at each edge and the middle of each side.

Diagnostic questions

• Tone or sharpness? Loupe first; everything else depends on the answer.
• Does the blank field show it? One sheet splits the causes into two groups that share nothing.
• Symmetric about the centre of the printing area, or not? Symmetric is geometry — depth, overhang, falloff. Asymmetric is something seated, fed or built wrong.
• Worse on 35 mm strips than on a flat sheet negative in the same frame? Curl, and the commonest sharpness fault on a proof sheet.
• Is the sheet mirrored, or does the fault stop at a rectangle smaller than the sheet? The first is printing through the base; the second is the overhang, the frame’s rebate, or tape around the glass.
• The same on the first and last sheet of the session? If it changes through the evening it is session exposure drift, a different entry and a different fix.
• On a printing-out process, did you open the back to inspect? Ware has you judge the simple cyanotype by eye through a hinged back, and a half put back down out of register, or with its springs unevenly reseated, prints the rest of the exposure through a different gap.

Corrective action

Work mechanical, then optical, then photographic — the build page’s commissioning order, so that no measurement is corrupted by an uncorrected fault beneath it.

Pressure and loading first. T1 names the failure: one edge is a weak spring or an unseated latch; the middle is glass or back bowing, answered by thicker glass, a batten across each half, or a fifth spring; everywhere is set foam or a cupped baseboard. Use closed-cell foam cut 5 mm smaller than the printing area so it cannot creep out and hold an edge open. Then flatten negatives under weight in their sleeves, sweep the glass, close the frame the moment it is loaded, and check orientation: emulsion to emulsion, negative facing the source, paper shiny side up.

Then the glass. Three millimetres is the working answer for a sprung 8 × 10 frame, or Ware’s 4 mm plate if you clip rather than spring; buy float cut and polished rather than salvage a sheet whose flatness you cannot check. Move to anti-Newton glass only once T2 has shown that rings actually print, its texture being in the light path and printing too. Under a UVA array, glaze in glass: ICNIRP record that window glass passes down to about 310 nm while most plastics, polymethyl methacrylate and polycarbonate among them, normally do not transmit below about 370 nm — a cut-off inside UVA and above the 365 nm LED peak.

Then the box: raise the diffuser, increase the overhang, seat the diffuser, feed a long strip from both ends. Each costs output, so change one and re-map; change three, get a good map, and you have learnt nothing about which mattered.

What you cannot usefully do is print around it. On an enlargement you burn the corners; here the sheet is exposed all at once under a closed frame, and the equation that condemns the gap condemns the dodge — a card 10 mm above the paper, under a source of aspect value near 1, casts a 10 mm penumbra. Fix the field, and write the correction on the label with the worst-point deviation, the paper and the date.

Prevention

Commission the printer before printing with it, in the build page’s order: T1 for pressure, T2 for rings, T3 for the field. A printer that has not passed those is a box with a lamp in it — and the frame goes against all three battens every time, or an exposure recorded in March means nothing in September.

Map once per geometry and keep the maps in the calibration records worksheet, re-mapping after any change to diffuser height, strip, glass or frame. A map turns “the corners look dark” into a figure in density, the only form in which this year’s fault can be compared with last year’s.

Keep to Kodak’s proof-sheet convention — expose so the clear rebate just reaches maximum black, and double or halve the time on the evidence of the processed sheet. A sheet exposed to a fixed convention is comparable with the last and the next, so a field going slowly wrong announces itself; one exposed until it looks right hides exactly that. Where a frame is not needed at all, ILFORD’s own paper-negative instruction presses a washed and dried print face to face with a fresh sheet under glass — close to a sprung frame for a flat sheet negative, and it is the curled 35 mm strip that makes the frame indispensable.