Thin negative from wrong reciprocity data

Lifted from Break/Fix — unexpectedly soft images and wrong reciprocity corrections, case 7, error 1.

What you see

A thin negative with empty shadows and a normal rebate. The unexposed edges are at ordinary base plus fog, so nothing is wrong with the material, the safelight or the developer; the picture simply did not get enough light.

The whole scale sits low. There is highlight detail and there is nothing at all where the shadows should be.

Likely causes

1. A correction taken from the wrong manufacturer’s sheet. This is a real number, correctly applied, to the wrong material — which is why it survives every arithmetic check.
2. No correction applied at all to an exposure well beyond one second.
3. A remembered exponent used in place of the published one.

What is happening: the chemistry and physics

Below some illuminance a material needs more total exposure than E = I × t predicts, because sparse photoelectrons fail to build stable development centres before the earlier ones decay. Manufacturers publish the compensation, and they do not publish it in the same form or with the same values.

🧮One metered time, two makers, and a factor of 4.4

Take a metered exposure of 10 seconds on sheet film.

FOMAPAN 100. FOMA publish a multiplier table rather than an exponent — at a metered 10 seconds the lengthening factor is ×8, so the correct exposure is 80 seconds.

FP4 PLUS. ILFORD publish T_c = T_m^P with P = 1.26 for FP4 PLUS, so T_c = 10^1.26 = 18.2 seconds.

Put FOMAPAN in the holder and ILFORD’s exponent in the notebook and you give 18.2 s where 80 s was needed

• a factor of 4.4, which is 2.14 stops underexposed. On a film with FOMAPAN’s latitude the shadows are gone.

ILFORD also note that exposures of one second or less need no compensation, and that contrast is increased with long exposures so that pulling the development may be required — so an uncorrected long exposure is usually thin and contrasty, which is a second clue.

Diagnostic questions

• Recompute from the maker’s sheet named in the log. The log has a field for the sheet the correction came from, and this fault is why.
• What is the ratio between what you gave and what the correct sheet asks for? The answer is usually a factor of two, four or five and a half, and each of those has a name. A factor near 4.4 with two different makers’ materials in play is this entry.
• Is the rebate normal? It must be, or the fault is not exposure. A fogged rebate points at the box, the safelight or the developer.
• Was the metered time over one second? Below that, no correction was needed and this entry does not apply.
• Compare with the over-corrected case, which is the same mistake in the other direction.

Corrective action

Nothing on the negative. An empty shadow has no information in it to bring back, because that part of the subject never made a developable centre at all.

Re-shoot with the correct sheet for the material in the camera, and note that you mostly cannot tell this fault from the negative alone — 2.14 stops under and 2.53 stops under look much the same. The log is what separates them.

Prevention

Use the sheet for the material in the camera, and no other. That is the instruction the exposure lesson gives and it is the whole of the prevention.

Log the sheet and its edition, not just the number you took from it. A manufacturer’s sheet is revised, and a correction from a superseded edition is a different fault with the same appearance.

Record the metered time and the corrected time as separate fields, so that the correction can be checked without reconstructing it.

Where you are working with a material whose reciprocity data you do not have, bracket widely and say so in the log rather than borrowing a number from a material you do have.