Blocked shoulder

Lifted from the characteristic curve, whose anatomy section names the region, and from gamma, contrast index and average gradient.

What you see

The top of the curve bends over sooner than it should, so the last few steps of the wedge are closer together in density than the steps below them. Plotted, the straight line ends early and the shoulder takes over.

On the negative this is blocked highlights — dense, detail-free skies and lit walls — and that entry describes what the eye sees. This entry is the measurement, and the measurement says something the eye cannot: where on the exposure axis the blocking begins, which is what distinguishes a film that was over-exposed from a film that was over-developed.

Likely causes

1. Over-exposure. The subject was placed too high on the axis, so its highlights landed past the end of the straight line. The whole curve is where it always was; the picture is not.
2. Over-development. The upper scale rises fastest with development, so a curve developed too far reaches the ceiling of the coating sooner and shoulders earlier.
3. A subject longer than the film’s straight line at that development. Nothing is wrong; the scene had more range than the material was set up to hold.
4. A film that shoulders by design. Some curves are made this way and are used deliberately for it.

What is happening: the chemistry and physics

Kodak’s workbook puts the shoulder as the region where the slope falls away and further exposure buys less and less density. Part XIII gives the reason in two parts and states it as the standard interpretation rather than as a measurement: fewer crystals remain to recruit, and the silver already laid down shields what is below it. Underneath both sits a hard ceiling — a film carries a finite mass of silver halide per unit area, and a developed image cannot be denser than that mass and its covering power allow.

That ceiling is why over-development and over-exposure produce the same appearance by different routes. Over-exposure moves the subject towards the ceiling. Over-development moves the ceiling towards the subject.

!The confirming test, and it is the one thing the negative cannot tell you

Read the shadow end of the same strip.

• Shadows also higher than aim, and D-min normal: the exposure was too great. Everything moved together.
• Shadows at aim and only the upper steps compressed: the development was too far. The bottom of a curve barely moves with development time and the top moves a great deal, which is precisely why contrast index is a development measure.

One reading, two numbers, and the two causes that look identical on a light box separate cleanly.

Diagnostic questions

• Does the strip reach the shoulder at all, or has the wedge run out? Kodak note that a black-and-white curve often never reaches D-max because a step tablet’s three log units run out first. Stouffer’s T2115 covers 3.05 of density and the T4110 covers 4.05; a curve that “shoulders” at the last step of a short wedge may simply have ended.
• What was the subject’s luminance range? A range longer than the straight line has to be compressed somewhere, and choosing where is the point of tone reproduction.
• What does the published curve for this film at this development look like? Compare shapes before concluding your process bent one.
• Was the contrast index also high? A high index and an early shoulder together is over-development with two witnesses.

Corrective action

For the negative you have, print it with the highlights placed where the paper can hold them, and accept the compression. A shoulder is lost separation, not lost density: there is nothing behind it to recover.

For the process, apply the classic asymmetry — expose for the shadows, develop for the highlights. Reduce development to bring the upper scale down; reduce exposure to move the whole subject down the axis. They are different remedies for different faults and this entry’s confirming test is what tells you which.

Reduce in the size of step a manufacturer uses: Kodak move contrast index towards aim in 10 per cent increments of development time.

Prevention

Measure the subject rather than guessing it. A meter reading of the brightest and darkest places you want detail in gives the range you have to fit, and Part XIII’s tone-reproduction page is where fitting it is worked.

Know your own film’s straight line at your own development. The step-wedge exposure series is the experiment that establishes it, once, for the combination you actually use.

Use a wedge long enough to see the shoulder if the shoulder is what you are studying. A 21-step wedge at 0.15 is a fine general instrument and it stops at 3.05.