Cut-off toe

Lifted from the characteristic curve and film speed and exposure index.

What you see

The curve starts too suddenly. Instead of easing up out of base-plus-fog over a log unit or so, the first step that shows any density at all is already on the straight line, or nearly so. Plotted against the wedge, the toe is short or missing, and the steps below it are all at D-min.

On the negative this is empty shadows: clear film where there should be detail. This entry is what the plot adds — it says whether the shadows are thin or absent, and those are different faults with different remedies.

Likely causes

1. Under-exposure. The subject sat too low on the axis and its shadows fell below the threshold exposure.
2. A speed rating the film does not deliver in this developer. Effective film speed is a property of the pair, not of the box.
3. The instrument. An exposing lamp or a wedge whose lowest steps deliver less than the nominal figure truncates a toe that the film has.
4. A genuinely short toe. Some films have one, and a short toe is not a fault.

What is happening: the chemistry and physics

Part XIII gives the standard interpretation, and states it as such: the toe is a picture of the spread of sensitivity across the grains. At very low exposure only the largest and most efficiently sensitised crystals have accumulated a latent-image centre big enough to catch a developer, so few develop and density rises slowly. As exposure increases, more of the population crosses the threshold and the slope steepens. A wide spread gives a long gradual toe; a narrow spread gives a short abrupt one.

Cut the exposure and you cut the bottom off that recruitment. Nothing crosses the threshold, so nothing develops, so there is no density — and no density is not a small density that a printer can lift.

!Why this is the fault that cannot be printed around

Part XIII states the asymmetry precisely and it is the reason exposure advice is geometry rather than folklore.

On the straight line, halving the exposure costs a fixed density difference and the tones stay separated. On the toe, halving the exposure costs less density difference, because the slope there is lower — so the shadows do not merely get thinner, they get flatter, and two shadow tones that were distinguishable merge.

No printing decision recovers a separation that was never recorded. A blocked shoulder loses highlight separation the same way, but a photographer can usually choose to place highlights; shadows are where the subject puts them.

Diagnostic questions

• Which step is the first above D-min, and by how much? With a 0.15 wedge, each step is half a stop. Counting steps from the first rise to the aim step converts the fault directly into an exposure correction.
• Was D-min measured on this run? The speed point is defined as a fixed density above base plus fog, so it moves with base plus fog. Read against last month’s D-min you will get last month’s answer.
• Is contrast at aim? Shadows low with contrast at aim is exposure. Shadows low with contrast low as well is development, and the fault is a low contrast index with the toe as a symptom.
• What exposure index were you using, and where did it come from? ISO speed is determined at a specified development contrast; your developer is probably not that one.
• Has the exposing instrument been checked? Part XIV’s calibration lesson exists because an instrument that has drifted moves every point on every curve you plot with it.

Corrective action

For this negative, very little. Extending development raises the upper scale far more than the lower one, so it buys contrast and not shadow detail; the empty steps stay empty. Intensification is a different subject and the course has no assessed intensifier to hand you.

For the next exposure, add light. The step count above converts to stops directly. If shadows fell two steps below aim on a 0.15 wedge, that is one stop.

If the index is the problem, change the index and record it. An effective exposure index established for your film in your developer is worth more than the box speed, and Part XIII is where you establish it.

Prevention

Expose for the shadows. It is one sentence and this entry is the argument for it.

Establish your own speed point once, with the step-wedge exposure series, and then use it.

Measure base-plus-fog on every run, on a strip of the same film processed alongside. It is the course’s convention and it is what keeps a speed reading comparable from month to month.