How does photographic film work?
Many materials are sensitive to light. Leave a piece of white office paper in your window for a few weeks and you might well find it turns yellow; plastics that start off white or clear also have a habit of turning yellow or going foggy (“photodegrading”) when they’ve been exposed to light for a while. The dyed colors in cotton clothes and fabrics will also fade in sunlight. And if you’re Caucasian, even your skin may change color after a few hours or days on the beach. But you can’t really use paper, plastic, cotton, or skin to capture a picture!
Photographic film is plastic (or sometimes paper) that’s coated with an emulsion made from microscopically tiny crystals of silver salts suspended in gelatin (a jelly-like substance found in sweets such as wine gums). The silver salts are compounds of silver and halogens such as chlorine, iodine, and bromine, also called silver halides—and their useful feature is the way they begin to change into pure, metallic silver when light falls onto them. If lots of light hits them, they change much more dramatically than if less light hits. This is how the two-dimensional pattern of light rays entering through the lens of a camera from the world outside forms a kind of invisible, chemical trace (called a “latent” image) on the surface of photographic film.
A light-sensitive slice of plastic film with an image invisibly imprinted on it isn’t much use to anyone. To turn it into a recognizable photo, you have to develop the film in a darkroom (usually lit with red or green light that doesn’t affect the film). This involves dipping the film in a series of chemicals, which convert the latent image captured by the tiny silver halide crystals into a visible image formed of larger silver particles, and also makes that image permanent.
First, the film is dipped in an acidic solution called developer, which encourages more of the silver halide to convert to metallic silver and renders the latent image visible. To stop this process continuing indefinitely, and ruining the photo, the film then has to be dipped in an acidic solution called a stop bath to neutralize the developer. Once that’s done, the image is made permanent by dissolving any remaining silver halide using a chemical solution known as hypo (or fixer), before being rinsed clean in water and hung up to dry.
At this stage, the image, though visible, is still in a negative pattern, with light areas looking dark and vice versa. That’s why developed pieces of film are called negatives. Once the film is developed, it’s printed: broadly speaking, you shine a light through the negative so it casts a shadow onto photo-sensitive paper and turns the negative film into a recognizable photograph called a (positive) print. You can make any number of prints from a single negative, which is one of the great advantages of this slightly laborious, “positive-negative” photographic process. By adjusting the distance between the negative and the paper you’re printing on, and using lenses, you can also enlarge or reduce the size of a an image. The piece of equipment you use to do this is called an enlarger.
1: Exposure captures an inverted (upside down) latent (invisible) image on the film inside your camera.
2: Developing uses a series of chemicals to make the latent image visible and fix it permanently on the film in the form of a negative.
3: Printing produces a final photograph (a positive print) from the negative. You can make any number of prints from one negative.
It’s possible to develop and print films yourself, but most photographic laboratories have large electronic machines that automate the process completely, threading the film through a series of tanks filled with chemicals in the correct sequence, at just the right speed. Those big photo-printing machines you still sometimes see in the back of drug stores typically use a method of developing color film called the C-41 process.
Many materials are sensitive to light. Leave a piece of white office paper in your window for a few weeks and you might well find it turns yellow; plastics that start off white or clear also have a habit of turning yellow or going foggy (“photodegrading”) when they’ve been exposed to light for a while. The dyed colors in cotton clothes and fabrics will also fade in sunlight. And if you’re Caucasian, even your skin may change color after a few hours or days on the beach. But you can’t really use paper, plastic, cotton, or skin to capture a picture!
Photographic film is plastic (or sometimes paper) that’s coated with an emulsion made from microscopically tiny crystals of silver salts suspended in gelatin (a jelly-like substance found in sweets such as wine gums). The silver salts are compounds of silver and halogens such as chlorine, iodine, and bromine, also called silver halides—and their useful feature is the way they begin to change into pure, metallic silver when light falls onto them. If lots of light hits them, they change much more dramatically than if less light hits. This is how the two-dimensional pattern of light rays entering through the lens of a camera from the world outside forms a kind of invisible, chemical trace (called a “latent” image) on the surface of photographic film.
A light-sensitive slice of plastic film with an image invisibly imprinted on it isn’t much use to anyone. To turn it into a recognizable photo, you have to develop the film in a darkroom (usually lit with red or green light that doesn’t affect the film). This involves dipping the film in a series of chemicals, which convert the latent image captured by the tiny silver halide crystals into a visible image formed of larger silver particles, and also makes that image permanent.
First, the film is dipped in an acidic solution called developer, which encourages more of the silver halide to convert to metallic silver and renders the latent image visible. To stop this process continuing indefinitely, and ruining the photo, the film then has to be dipped in an acidic solution called a stop bath to neutralize the developer. Once that’s done, the image is made permanent by dissolving any remaining silver halide using a chemical solution known as hypo (or fixer), before being rinsed clean in water and hung up to dry.
At this stage, the image, though visible, is still in a negative pattern, with light areas looking dark and vice versa. That’s why developed pieces of film are called negatives. Once the film is developed, it’s printed: broadly speaking, you shine a light through the negative so it casts a shadow onto photo-sensitive paper and turns the negative film into a recognizable photograph called a (positive) print. You can make any number of prints from a single negative, which is one of the great advantages of this slightly laborious, “positive-negative” photographic process. By adjusting the distance between the negative and the paper you’re printing on, and using lenses, you can also enlarge or reduce the size of a an image. The piece of equipment you use to do this is called an enlarger.
1: Exposure captures an inverted (upside down) latent (invisible) image on the film inside your camera.
2: Developing uses a series of chemicals to make the latent image visible and fix it permanently on the film in the form of a negative.
3: Printing produces a final photograph (a positive print) from the negative. You can make any number of prints from one negative.
It’s possible to develop and print films yourself, but most photographic laboratories have large electronic machines that automate the process completely, threading the film through a series of tanks filled with chemicals in the correct sequence, at just the right speed. Those big photo-printing machines you still sometimes see in the back of drug stores typically use a method of developing color film called the C-41 process.