Low contrast index

Lifted from gamma, contrast index and average gradient and interpreting density data. The register assigns the curve-shape signatures to Parts XIII and XV rather than to Part XXVIII, and both of those pages are written, so this entry is a reading of them rather than an anticipation of anything.

What you see

On the plot, not on the light box. A curve whose straight line is measurably shallower than the one you aimed at: the same range of log exposure produces a smaller range of density. On a contrast index computed under the course’s convention, the number comes out below aim, and Kodak’s own acceptance window for a control strip is ±0.02, so a deviation of 0.05 or more is not a reading error.

On the negative itself the film looks grey and even, with detail everywhere and separation nowhere. That appearance is the subject of a separate entry — flat negative — and the distinction between the two entries is the whole reason this one exists. A flat negative is a judgement. A low contrast index is a measurement, and only the measurement tells you which of four causes you have.

Likely causes

1. Under-development. Time too short, temperature too low, agitation too sparse. This is the first hypothesis because slope is the thing development moves.
2. A developer that has lost activity. Oxidised in a part-filled bottle, or restrained by bromide released by the films already through it.
3. The wrong aim. The developer, the dilution or the film has a normal slope lower than the one you were expecting, and nothing is wrong at all.
4. The measuring chain. A wedge that is not what it claims, an exposing instrument whose steps are not the steps you think, or a densitometer compressing at the top of its range — each of which flattens a curve that the film never flattened.

What is happening: the chemistry and physics

Slope is the degree of development, and almost nothing else. Kodak’s workbook plots contrast index against development time and the shape is always the same: it climbs steeply at first, then bends over towards a limit set by the film and the developer. Read that curve backwards and a low index means you are too far to the left on it.

Two of the causes move you left by different routes, and the routes matter because the fixes differ.

Temperature is time. ILFORD’s published compensation chart makes 1 °C worth about 9 per cent of development, and Kodak state flatly that a developer varying by more than 0.3 °C affects process control and image quality. A bath two degrees cool has taken roughly a fifth off your development without touching the timer.

Bromide restrains, and it restrains progressively. ILFORD say it in a sentence — each film processed releases halides and other by-products which act as a restrainer on the development of subsequent films — and then price the remedy at 10 per cent more time for each successive film through a litre of stock ID-11, up to ten films. Part VIII owns the mechanism.

Oxidation is different again, because it costs the developer its reducing agents rather than merely slowing them. ILFORD give ID-11 stock six months full and one month half-filled; the oxidised developer entry is where the course records what that looks like in the bottle.

!The confirming test, and it is one strip

Process a second strip in fresh developer, one-shot, at a measured 20.0 °C, and read it.

If the index comes back to aim, the fault was in the bath or the bath’s temperature and the film is fine. If it does not, the fault is upstream of the developer — in the aim you chose, or in the measuring chain — and no amount of extra development will find it.

That single substitution splits four causes into two pairs in one run, which is why it comes before any adjustment of time.

Diagnostic questions

• What did the thermometer read, and when? Not what the tap was set to. A tank cooling towards room temperature spends most of its development somewhere other than where it started.
• How many films has this litre already done, and did you apply the compensation? ILFORD’s own caveat is that the 10 per cent figure can only be an approximation, because a roll of night shots consumes almost no developing agent and a roll of beach scenes consumes a great deal.
• When was the bottle mixed, and how full is it? The date and the headspace, together, are the oxidation test. Fog is not: fog moves little under oxidation, which is why the bottle rather than the D-min row settles it.
• Is the aim right? Read the developer’s own published slope before concluding the process is out. A dilution chosen for finer grain often costs contrast on purpose.
• Is the wedge calibrated? Stouffer’s specification table gives the T2115 as 21 steps of 0.15 to a D-max of 3.05, and marks separately which of its wedges are calibrated against NIST material. An uncalibrated wedge is a perfectly good relative instrument and a poor absolute one, and a curve plotted against nominal steps inherits every error in them.

Corrective action

For this negative, print it. A low-contrast negative is printable on a harder grade or a harder filter, and Part XIII’s tone-reproduction page is where the arithmetic of that trade lives. What cannot be recovered is separation that was never recorded, which is a toe problem rather than a slope problem.

For the process, change one thing. Kodak move contrast index towards aim in 10 per cent increments of development time; that is the size of step a manufacturer treats as worth taking, and it is a good size for you. Change the time, or the temperature, or the bath — never two together, because two changes leave you knowing only that something worked.

Do not chase a single point. Kodak’s warning about over-controlling a process by reacting to random variation applies here more than anywhere: two low readings are not a trend, and a fourth consecutive point on one side of aim is where a level shift becomes likely.

Prevention

Process one-shot. Dilute from stock immediately before use and discard afterwards. It removes bromide accumulation and aerial oxidation of the working bath at a stroke, and it is the single largest thing you can do to make your own contrast measurements interpretable. That is why the course processes one-shot on every sensitometric page.

Hold the temperature and write it down. A water bath, a thermometer read at the end as well as the start, and the actual number in the notebook rather than the intended one.

Keep a control chart. A slope that has drifted over six runs is invisible in any one of them and obvious on a plot, and the chart also tells you when a low reading is the process rather than the day.