Dense negative from double correction

Lifted from Break/Fix — unexpectedly soft images and wrong reciprocity corrections, case 7, error 2.

What you see

A dense negative with blocked highlights and a normal rebate. The unexposed edges are at ordinary base plus fog, so the material and the processing are exonerated; the picture got too much light.

A blocked highlight is an area that was bright in the subject and is now so dense that no detail can be seen in it. It is not recoverable — there is no information in it to bring back, because the material stopped responding before the exposure ended.

Likely causes

1. The correction was applied to a time that had already been corrected.
2. Two passes over the same log row, by two people or by the same person at two moments.
3. A corrected time written into the field meant for the metered time, so that the next step corrected it again.

What is happening: the chemistry and physics

The published relation is a function of the metered time and is applied once, to the whole of it. Applying it to its own output raises the metered time to the power of the exponent twice over.

🧮10 seconds, corrected twice

FP4 PLUS, P = 1.26.

Metered 10 s → corrected once, 10^1.26 = 18.2 s. Correct that again and 18.2^1.26 = 38.7 s, which is 1.09 stops over.

This is the error that hides best, because every individual step was done correctly. The arithmetic is right, the exponent is right, the material is right. Only the count is wrong.

The size of the over-exposure grows with the metered time, because the exponent compounds — so the fault gets worse exactly where reciprocity corrections matter most, on the very long exposures.

There is a second-order consequence worth knowing. ILFORD note that contrast is increased with long exposures, so that pulling the development may be required. A doubly corrected exposure is therefore dense and contrasty, and the two together are what produce the blocked highlight rather than merely a heavy negative.

Diagnostic questions

• Recompute from the metered time, not from the time in the exposure column. If applying the exponent once to the metered time gives you a number you did not use, and applying it twice gives you the number you did, the diagnosis is made.
• Is the over-exposure about one stop for a ten-second metered time, and more for longer ones? The signature grows with the metered time, which distinguishes it from a flat error such as a misread meter.
• Is the rebate normal? It must be.
• Does the log distinguish the metered time from the corrected time? If it has one time field, this fault is invisible by construction.
• Compare with the under-corrected case.

Corrective action

Nothing on the negative. A blocked highlight cannot be printed through, because there is no detail in it.

A dense negative that is not fully blocked can be printed on a softer grade with a long exposure, so it is worth trying before the frame is written off — which is the asymmetry the commissioning session teaches, that a slightly dense highlight can still be printed and an empty shadow cannot be printed at all.

Prevention

Apply the correction once, to the whole metered time, as the last step of the exposure calculation. The order matters, because the correction is a function of the total time.

Record the metered time and the corrected time as separate fields. That single change to the log makes this fault visible in the field rather than three weeks later.

Strike through and annotate rather than erase when a row is corrected, so that a row which has been worked twice says so on its face.

Where a reciprocity correction has been made, write which step of the calculation it was, so that a second person picking up the sheet can see it has been done.