Part XIII Overview: Measuring the Response of Silver to Light

“The negative looks a bit thin.” Everybody says it, nobody can act on it, and two photographers saying it about the same strip may be describing two different faults. Thin where — in the shadows, or everywhere? Thin because too little light reached the film, or because development was cut short? Thin by how much, and is that enough to matter?

Sensitometry answers by refusing the adjective. It puts a measured quantity of light into the film, measures how dark the film became, and plots one against the other. What comes out is not an opinion about a negative; it is a curve, and two people looking at the same curve are looking at the same thing.

The two axes, and why both are logarithms

Everything in this part sits on one diagram with two axes.

Along the bottom is exposure, written H: illuminance multiplied by time, in lux-seconds. Up the side is density: the base-ten logarithm of the reciprocal of transmittance, so that a density of 1.0 passes a tenth of the light and 3.0 passes a thousandth.

Both axes are logarithmic for reasons that are arithmetic rather than aesthetic. A subject can span thousands to one and a curve has to fit on a page; and densities add where transmittances multiply, which is why a step wedge works and why the sums here can be done in your head. Part III taught the base-ten logarithm once, for pH, and promised not to teach it twice. The first lesson keeps that promise.

The map

Axes come first. Then the curve itself — Hurter and Driffield’s plot, read on 31 May 1890 to the Liverpool section of the Society of Chemical Industry, with its base-plus-fog, its toe, its straight line and its shoulder. Then slope, which is contrast, in the three constructions the industry actually uses. Then speed, which is position along the exposure axis. Then an assignment that makes you do all of it by hand on supplied data, and a lab that produces your own first measured curve. Then paper, which has a curve of its own in reflection density, and finally tone reproduction, where subject, negative, paper and eye are put back into one diagram.

What you produce

Two things, and both are keepable. The first is an interpreted set of curves: plots, a speed point, a contrast figure for each condition, and a one-page recommendation with numbers and uncertainties in it. The second is a physical object — a processed step-wedge negative, with its unexposed control strip, archived alongside the data sheet that says what film, developer, dilution, temperature, time and agitation made it. Part XV’s densitometer will read that strip again, properly, and the two readings will be worth comparing.

What this part does not do

It does not build instruments: the sensitometer is Part XIV and the densitometer Part XV, and until they exist this part reads densities by interim methods and says what those methods can and cannot resolve. It does not run process control, which is Part XIV. And it does not explain the mechanism of reciprocity failure, which Part IV owns; here it appears only as a curve that moves.

What you need first

School arithmetic and the base-ten logarithm from Part III. The latent image and development as amplification from Part IV, because the curve is a picture of both. One film processed to completion. And experimental design for the darkroom from Part IX, which established the step wedge, the control strip and the uncomfortable fact that a wedge sold with nominal 0.15 increments is not a calibrated instrument.

Safety, in one paragraph

There is one chemical session in this part. The step-wedge lab processes film in a standard developer, stop and fixer at Level A, exactly as Part VI and Part IX did, and cites those procedures rather than restating them. Everything else here is pencil, paper, a calculator and a light box.

!How this course cites the standards

The conventions of sensitometry are written down in International Standards that are sold rather than published, and buying one would not grant the right to reproduce it. So the course does not purchase them. It states its own convention explicitly, on the page where it is first needed; it cites each standard by number as the thing that convention is modelled on; and it marks every figure derived under it as the course’s own measurement under the course’s own criterion.

So a page here may say “measured under the criterion defined in Part XIII, which is modelled on ISO 6”. It may never say “the ISO speed is”. That is not pedantry: a published criterion you can check beats an appeal to a document you cannot read.

How precise this can be, honestly

A home sensitometry chain measures relative speed and contrast well and absolute speed not at all. The reasons stack up: the exposing light is not a specified sensitometric illuminant, the densitometry is not the standard geometry, and the process is your own. What survives all of that is comparison — this developer against that one, this time against that time, your film against your own film last month — and comparison is what changes a decision in the darkroom.

Manufacturers concede as much. ILFORD rates HP5 Plus on a practical evaluation of film speed and says outright that this is not the foot speed the ISO standard uses. The number on the box is one honest answer to a question that has several.

The pages of this part

Part13 of 28Level2 — PractitionerPages8Estimated time11.0 hoursHighest safety levelLevel A

✱1 of 8 pages in this part need a darkroom, a UV source or mains-powered equipment, marked below. Each says what can be improvised and, where one exists, gives an alternative route.

0 / 8 lessons in this part completed

• LessonExposure, Density and the Logarithm60 min
• LessonThe Characteristic Curve60 min
• LessonGamma, Contrast Index and Average Gradient60 min
• LessonFilm Speed and Exposure Index60 min
• AssignmentAssignment: Interpreting Density Data120 min
• LabLab: A Step-Wedge Exposure SeriesA✱ Darkroom · Mains180 min
• LessonPaper Sensitometry: Reflection Density, Grades and ISO(R)60 min
• LessonTone Reproduction: From Subject to Print60 min