# Film and the Characteristic Curve

Photography I · The Black-and-White Analogue Laboratory · https://tryals.app/learn/photography-i/film-and-the-characteristic-curve

## Density Against Log Exposure

Ferdinand Hurter and Vero Driffield published the sensitometry that still governs film in **1890**. Their graph plots **density** against the **logarithm of exposure**.

**Density** is the base-ten logarithm of opacity. Transmitting one tenth of the light gives a density of 1.0. Transmitting a hundredth gives 2.0. **One stop is 0.30 in log exposure**.

The curve has four main regions:

| Region | What it is | What it renders |
|---|---|---|
| **Base plus fog** | Minimum film density | Clear film, the floor |
| **Toe** | Rising lower curve | Compressed deep shadows |
| **Straight line** | Constant-slope middle | Evenly separated mid-tones |
| **Shoulder** | Flattening top | Compressed highlights |

The slope of the straight-line section is the **gamma**. A steeper slope gives higher contrast. Gamma is **not a fixed property of the film**, as it rises with development time. Manufacturers often quote a **contrast index**, measuring the average usable gradient instead.

### Film Speed and Grain

The ISO speed of black-and-white film is fixed on the toe. The speed point requires a density of **0.10 above base plus fog**.

Underexposure destroys shadow detail first. Therefore, the classic rule is to expose for the shadows.

| Film speed | Crystal size | Grain | Resolving power |
|---|---|---|---|
| ISO 25 to 50 | Small | Very fine | Very high |
| ISO 100 to 400 | Medium | Moderate | Good |
| ISO 1600 to 3200 | Large | Coarse | Lower |

Larger crystals catch more photons, making them faster. They also leave larger grains of silver. **Tabular-grain** emulsions use flattened crystals to improve this trade-off.

> **Common pitfall:** Treating the box speed as a property of the film alone. It only holds for film developed to a specified contrast.

## Practice questions

8 of this lesson's 15 practice questions, with answers. The full set is in the app.

### 1. Two negatives of the same scene are developed for different times, so their curves have different slopes. How does that slope translate into what the print eventually looks like?

A. A steeper slope shifts the straight line up, giving denser shadows with compressed highlights
B. A steeper slope increases effective ISO speed, recording shadow detail with far less exposure
C. A steeper slope changes only base fog, which standard darkroom paper filters readily correct
D. A steeper slope yields greater density range per subject luminance, increasing print contrast

**Answer:** D. A steeper slope yields greater density range per subject luminance, increasing print contrast

**Why:** Slope *is* contrast, in the literal sense: it is how much density the negative gains per stop of subject brightness. A steep curve turns a small brightness difference into a large density difference, which the paper turns into a large tonal separation. Nothing at the printing stage corrects it; the paper grade only decides how much of it fits.

Page: https://tryals.app/practice/photography-i/film-and-the-characteristic-curve/two-negatives-of-the-same-scene-are-developed-for-different-times-so

### 2. A part of a negative transmits one hundredth of the light falling on it. What is its density?

**Answer:** 2

**Why:** **2.0.** Opacity is 100, and the base-ten logarithm of 100 is 2. A density of 2.0 is about as dense as the highlights of a normal negative go, which is worth remembering: the whole useful range of a black-and-white negative is roughly 0.1 to 1.4 above base plus fog.

Page: https://tryals.app/practice/photography-i/film-and-the-characteristic-curve/a-part-of-a-negative-transmits-one-hundredth-of-the-light-falling-on

### 3. Complete the vocabulary of sensitometry.

**Answer:** The curving foot of the characteristic curve, where the deep shadows sit, is the **toe**. The flattening top, where the highlights are compressed, is the **shoulder**. The slope of the straight-line portion is the **gamma**. One stop corresponds to a log exposure interval of **0.30**.

**Why:** The last blank is the conversion that makes the whole graph usable. Once a stop is 0.30 in log exposure, a seven-stop scene occupies 2.10 on the horizontal axis, and you can read straight off the curve whether the film will hold it.

Page: https://tryals.app/practice/photography-i/film-and-the-characteristic-curve/complete-the-vocabulary-of-sensitometry

### 4. The ISO speed point for a black-and-white negative film is measured on the straight-line portion of the characteristic curve.

**Answer:** False

**Why:** **False**: it is on the **toe**, at a density of 0.10 above base plus fog. That is precisely why the speed rating is a statement about shadows: it marks the least exposure that produces any usable separation at all, which is why underexposure empties the shadows before it touches anything else.

Page: https://tryals.app/practice/photography-i/film-and-the-characteristic-curve/the-iso-speed-point-for-a-black-and-white-negative-film-is-measured

### 5. Sort each description by the region of the characteristic curve it belongs to.

**Answer:**

- Toe: Compressed deep shadows, ISO speed defined
- Straight-line portion: Evenly spaced mid-tones, Gamma measured
- Shoulder: Flattened highlights, Negligible density gain

**Why:** The two compressed regions are not defects. Shoulder compression is what lets a negative hold a highlight several stops beyond what a straight response could accommodate, and it is the main reason negative film survives overexposure so much better than a digital sensor does.

Page: https://tryals.app/practice/photography-i/film-and-the-characteristic-curve/sort-each-description-by-the-region-of-the-characteristic-curve-it

### 6. Exposure sets the position of tones along the log-exposure axis, while development controls the slope of the straight-line portion. Why must a photographer expose for the shadows rather than compensating for underexposure in development?

A. Development cannot retrieve unrecorded toe values
B. Prolonged processing only lifts highlight contrast
C. Underexposure causes an irreparable shift in gamma
D. Shadow density is governed entirely by base plus fog

**Answer:** A. Development cannot retrieve unrecorded toe values

**Why:** Development increases curve gradient across the mid-tones and highlights, but tones falling below the toe threshold receive no exposure distinction to amplify. Shadow values depend on initial photon capture rather than chemical intensification.

Page: https://tryals.app/practice/photography-i/film-and-the-characteristic-curve/exposure-sets-the-position-of-tones-along-the-log-exposure-axis

### 7. Which are true of the way a black-and-white negative film's ISO speed is determined?

A. The standard also specifies the development, so the quoted speed assumes it
B. The speed point is the exposure giving a density of 0.10 above base plus fog
C. The speed is measured at the steepest part of the curve
D. The speed point sits in the shadows, which is why underexposure empties them first

**Answer:** A. The standard also specifies the development, so the quoted speed assumes it; B. The speed point is the exposure giving a density of 0.10 above base plus fog; D. The speed point sits in the shadows, which is why underexposure empties them first

**Why:** The fourth confuses the two measurements. Speed is fixed low on the toe and contrast is measured on the straight line, which is why the two can be varied almost independently, the basis of the entire zone system.

Page: https://tryals.app/practice/photography-i/film-and-the-characteristic-curve/which-are-true-of-the-way-a-black-and-white-negative-films-iso-speed

### 8. On the straight-line portion of a film whose gamma is 1.0, a density difference of 0.30 corresponds to how many stops of exposure?

**Answer:** 1 (within ±0.3)

**Why:** **One stop.** With a gamma of 1.0 the density difference equals the log exposure difference, and one stop is 0.30 in log exposure. Real black-and-white negatives are developed to a gamma nearer 0.55 to 0.65, so a stop of subject brightness produces only about 0.17 of density.

Page: https://tryals.app/practice/photography-i/film-and-the-characteristic-curve/on-the-straight-line-portion-of-a-film-whose-gamma-is-1-0-a-density