Film — The Chemistry of Silver

In the preceding three lessons we have assembled the mechanics of capturing light: the aperture controls intensity, the shutter controls duration, and a meter tells us how much light is present. But all of this machinery serves a single purpose — to deliver a precise quantity of photons to a thin layer of chemistry that can record them. That layer is the photographic emulsion, and its active ingredient is one of the most light-sensitive substances known: silver halide.

For nearly two centuries, from the first permanent photograph in the 1820s to the film rolls you can buy today, the fundamental chemistry has remained the same. Crystals of silver halide, suspended in gelatin, respond to light by forming an invisible latent image — a chemical whisper that is later amplified billions of times by development. Understanding how this process works at the molecular level transforms film from a mysterious black box into something you can reason about, predict, and control.

Silver Halide Crystals

The light-sensitive materials in photographic film are silver halide crystals — compounds of silver with one of the halogen elements: chlorine, bromine, or iodine. In practice, most photographic emulsions use primarily silver bromide (AgBr), often with a small percentage of silver iodide (AgI) mixed in. Silver chloride (AgCl) is used in photographic papers but rarely in film, because it is less sensitive to light.

These crystals are grown in a solution of gelatin — the same protein derived from animal collagen that gives Jell-O its texture. Gelatin is not merely a convenient binder. It plays an active chemical role: sulfur compounds naturally present in gelatin create tiny clusters of silver sulfide on the crystal surfaces. These clusters, only a few atoms across, are called sensitivity specks, and they are essential to the formation of the latent image.

The size, shape, and uniformity of the silver halide crystals determine much of a film's character. Large crystals are more sensitive to light (faster film speed) but produce visible grain in the final image. Small crystals are less sensitive (slower film speed) but yield finer grain and higher resolution. The art of emulsion making — one of the most closely guarded industrial secrets of the twentieth century — lies in controlling crystal growth to achieve the desired balance of speed, grain, and sharpness.

How Light Creates the Latent Image

The latent image is the invisible record of exposure that exists in the film after it has been exposed to light but before it has been developed. Its formation is a quantum mechanical process, and understanding it requires thinking at the level of individual photons, electrons, and atoms.

The process unfolds in several steps:

1. Photon absorption. A photon of visible light strikes a silver halide crystal and is absorbed. The photon's energy (about 2 to 3 electron-volts for visible light) is transferred to a bromide ion (Br−) in the crystal lattice, knocking loose an electron. The bromide ion becomes a neutral bromine atom.
2. Electron migration. The freed electron, called a photoelectron, moves through the crystal lattice until it encounters a sensitivity speck — one of those tiny silver sulfide clusters on the crystal surface. The speck traps the electron.
3. Silver ion attraction. The trapped electron gives the sensitivity speck a negative charge, which attracts a mobile silver ion (Ag+) from the crystal lattice. The silver ion combines with the trapped electron to form a single atom of metallic silver (Ag0) at the speck.
4. Repetition and growth. As more photons strike the crystal, the process repeats. More electrons are trapped, more silver ions are attracted, and the cluster of metallic silver atoms at the sensitivity speck grows. A cluster of roughly four or more silver atoms constitutes a latent image center — a site that will catalyze the development of the entire crystal.

The extraordinary thing about this process is its amplification. During exposure, only a few atoms of metallic silver are created at each sensitivity speck — perhaps four to ten atoms. But during development, the chemical developer uses this tiny cluster as a catalyst to reduce the entire silver halide crystal to metallic silver. A single crystal may contain ten billion silver atoms. So the latent image is amplified by a factor of roughly a billion. No electronic amplifier in existence achieves this gain — yet it happens through simple chemistry, in a gelatin coating, at room temperature.

The Structure of Film

A strip of photographic film is not a single layer. It is a carefully engineered stack of coatings, each serving a specific purpose. From top to bottom (the side facing the lens is the top):

Overcoat (supercoat). A thin, clear layer of hardened gelatin that protects the emulsion from scratches and abrasion during handling. It must be hard enough to resist damage but transparent enough not to degrade the image.

Emulsion layer. This is the active photographic layer, typically 5 to 20 micrometers thick. It consists of silver halide crystals suspended in gelatin, along with sensitizing dyes, chemical sensitizers, and other additives. Some films have multiple emulsion sub-layers of different speeds to increase dynamic range. The crystals in a fast emulsion (ISO 400) are larger and more dispersed than in a slow emulsion (ISO 100).

Subbing layer (adhesion layer). A very thin layer that bonds the emulsion to the base. Without it, the gelatin emulsion would peel away from the plastic support.

Base (support). The transparent foundation, typically 100 to 180 micrometers thick. Modern films use either cellulose triacetate (a safety film introduced in the 1940s to replace the dangerously flammable cellulose nitrate used in early cinema) or polyethylene terephthalate (PET, marketed by Kodak as “Estar” base). PET is stronger, more dimensionally stable, and thinner, but cellulose triacetate remains popular because it curls less and is easier to handle in darkroom processing.

Anti-halation layer. Coated on the back of the base (or sometimes between the base and the emulsion), this layer absorbs light that passes through the emulsion without being captured. Without it, light would reflect off the base and scatter back up into the emulsion, creating halos around bright objects. The anti-halation layer is typically a dye or pigmented gelatin that is decolorized or washed away during processing.

Film Speed: ISO and ASA

Film speed describes how sensitive an emulsion is to light — how many photons are needed to form an adequate latent image. The modern standard is ISO (International Organization for Standardization), which combined two older systems in 1974: the American ASA (American Standards Association) arithmetic scale and the German DIN (Deutsches Institut fur Normung) logarithmic scale. In practice, most photographers use only the arithmetic (ASA-style) number: ISO 100, ISO 400, ISO 3200.

The arithmetic scale is linear: each doubling of the number means the film is twice as sensitive, requiring one stop less exposure. ISO 200 film is one stop faster than ISO 100. ISO 400 is two stops faster than ISO 100, and one stop faster than ISO 200. This direct correspondence with the stop system makes exposure calculations straightforward.

The DIN scale is logarithmic: each increase of 3 DIN equals one stop. ISO 100 is 21 DIN; ISO 200 is 24 DIN; ISO 400 is 27 DIN. The full ISO designation combines both: ISO 100/21°, ISO 400/27°. But outside of Germany and a few other European countries, the DIN portion is rarely used.

The Grain-Speed Tradeoff

There is an inherent tradeoff in emulsion design between speed and grain. Larger silver halide crystals have a greater probability of being struck by a photon during a given exposure (because they present a larger target) and therefore make the film faster. But those larger crystals also produce larger clumps of metallic silver after development, which are visible as grain in enlarged prints or scans.

This is why ISO 100 film (Kodak Ektar, Fujifilm Acros) produces smooth, fine-grained images, while ISO 3200 film (Kodak T-Max P3200, Ilford Delta 3200) has prominent, visible grain. For many photographers, grain is not a defect but a texture — an aesthetic quality that gives film its distinctive organic character.

T-Grain Technology

In 1986, Kodak introduced a revolutionary approach to this tradeoff. Traditional silver halide crystals are roughly spherical or irregularly shaped, but Kodak's research team developed a process for growing flat, tabular crystals — thin hexagonal plates that Kodak called T-Grains. These tabular crystals present a large flat face to incoming light (increasing the chance of photon capture) while being very thin (reducing the total silver mass per crystal). The result is better light capture per unit of grain — effectively, higher speed for a given grain size, or finer grain for a given speed.

The first T-Grain black-and-white films were Kodak T-Max 100 and T-Max 400, both introduced in 1986, followed eventually by T-Max P3200. Ilford responded with their own tabular-grain technology in the Delta line (Delta 100, Delta 400, Delta 3200). Fujifilm's Neopan Acros also used advanced crystal growth techniques. T-Grain technology represented one of the last great leaps in film emulsion science before digital photography shifted the industry's focus.

Color Film: Three Layers

Black-and-white film has a single emulsion layer (or sometimes two layers of different speeds for extended dynamic range). Color film is fundamentally more complex: it must record not just how much light struck each point, but also its color. To do this, color film uses three separate emulsion layers, each sensitized to a different region of the visible spectrum.

The top layer is sensitized to blue light. Below it, a yellow filter layer blocks blue light from reaching the lower layers (since all silver halide has some inherent blue sensitivity). The middle layer is sensitized to green light. The bottom layer is sensitized to red light.

Each layer contains silver halide crystals plus color couplers — organic dye precursors that react with the oxidized developer during processing to form colored dyes. The blue-sensitive layer forms yellow dye, the green-sensitive layer forms magenta dye, and the red-sensitive layer forms cyan dye. After processing, the metallic silver is bleached away, leaving only the dye images.

These are subtractive primary colors (cyan, magenta, yellow), which combine to reproduce the full range of visible color. The system was first commercialized by Kodak as Kodachrome in 1935 (where the couplers were in the developer rather than the film) and later refined into the Kodacolor and Ektachrome lines (with couplers incorporated directly into the emulsion layers).

Three Types of Film

Understanding the distinction between the three main categories of photographic film is essential for any photographer.

Black-and-White Negative

The simplest type. After development, exposed areas become dense (dark) with metallic silver, while unexposed areas are clear. The result is a negative image: lights and darks are reversed. To create a positive print, light is projected through the negative onto photographic paper, reversing the tones again. Common examples: Kodak Tri-X 400, Ilford HP5 Plus, Fujifilm Acros II.

Color Negative (C-41)

Functionally similar to black-and-white negative, but with three dye layers instead of one silver layer. The developed film shows reversed tones and reversed colors (a process called complementary color recording). Color negatives also have an overall orange appearance caused by colored masking couplers that improve color accuracy in the final print. All color negatives are processed in the standardized C-41 chemistry. Common examples: Kodak Portra 160, Kodak Portra 400, Kodak Gold 200, Fujicolor 200.

Color Reversal (Slide / Transparency / E-6)

Unlike negative films, reversal film produces a positive image directly on the film. After a first development step that creates a negative silver image, the film is chemically fogged (or re-exposed to light) to develop the remaining unexposed silver halide. Color couplers create dyes in this second development, and the original negative silver image is bleached away. The result is a positive transparency with correct colors and tones that can be projected directly or viewed on a light table. Standard processing is E-6. Common examples: Fujifilm Velvia 50, Fujifilm Provia 100F, Kodak Ektachrome E100.

Reversal film has much less exposure latitude than negative film. While a color negative can tolerate two or more stops of overexposure and still produce a usable image, reversal film requires exposure accuracy within about half a stop. This is why slide film rewards careful metering and punishes errors — and why the lessons on metering we covered earlier are especially important if you plan to shoot transparencies.

Edge Markings and DX Coding

The edges of every roll of film carry useful information. On 35mm film, edge markings printed by the manufacturer include the film name, emulsion batch number, and frame numbers. These markings are exposed onto the film during manufacturing by a light source at the coating plant, so they appear on every roll as part of the base.

In 1983, Kodak introduced the DX coding system for 35mm film cartridges. A pattern of conductive and insulating patches on the outside of the cartridge encodes the film speed, number of exposures, and exposure latitude in a machine-readable format. Cameras with DX-reading contacts (most 35mm cameras from the mid-1980s onward) automatically set the ISO based on the cartridge. This eliminated a common source of error — forgetting to set the meter to match the loaded film.

120 film, used in TLRs and other medium-format cameras, does not have DX coding. The paper backing of a 120 roll carries frame number markings visible through a small red window on the camera back, but there is no automatic speed detection. TLR photographers must remember to set their light meter to the correct ISO each time they change film stocks — a step that becomes second nature with practice.

The Material Miracle

It is worth pausing to appreciate what film accomplishes. A thin layer of silver crystals in gelatin, coated on a strip of plastic, captures a latent image using a quantum mechanical process that amplifies a handful of absorbed photons into a visible record of the world. The entire system requires no batteries, no electronics, no software. It works at any temperature where gelatin does not melt or freeze solid. A roll of unexposed film, stored properly, retains its sensitivity for years or decades. And the final silver image, once processed and fixed, is among the most archivally permanent imaging media ever created.

With this understanding of how film records light at the molecular level, we have completed Unit 2. In the next unit, we will step back in time to trace the history of photography itself — from the camera obscura of the Renaissance to the daguerreotype, the collodion process, and the invention of roll film that made portable cameras possible.