What is 35mm film made of?

by Chris Woodford. Last updated: August 26, 2022.

Snap a photo with your camera, cellphone, or MP3 player and you have a piece of digital information you can use in all kinds of different ways: you can instantly email it to a friend, upload it to a website, or edit it on your computer. Only a few years ago, this sort of thing wasn't possible because cameras worked differently: they were entirely analog, capturing pictures as patterns of light and dark using chemically treated reels of plastic film, and a photograph took hours (or even days) to appear in your hand. Although some professional photographers still use film cameras, most of us have long since abandoned them to history, along with steam engines and the telegraph. That's a shame, because they can actually teach us quite a bit about chemistry, physics, and the science of light. Let's take a closer look at how they worked!

Photo: A typical 35mm film camera dating from the mid-1990s. This one's made by Olympus and includes a motorized zoom lens that extends up to 80mm (about 3in). The plastic window at the top houses the autofocus system and the viewfinder and the white rectangle on the right (as we look at the camera) is the flash.

Contents

Articles like this typically open by noting that the word photography comes from two Greek words, photos (light) and graphos (writing)—so photography effectively means "writing with light." But that's just a metaphor. We see objects because they either emit light (like the Sun) or reflect it off their surface (like the Moon) in rays that zoom into our eyes in perfectly straight lines. Much like the human eye, a camera captures rays of light charging in through a lens at the front. But the crucial difference between a camera and your eye is that a camera makes a permanent copy of what it sees.

"Photography" is a bit of a misnomer: it's not really true to say that light rays "write" or "draw" in any sense. They don't move back and forth, thoughtfully scribbling out a picture like an artist's hand. So what does happen? It always helps to think like a scientist! Remember that light is a form of energy; what a camera actually does is permanently capture the energy falling on a small, (two-dimensional) surface inside it. In a digital camera, that happens because there's an electronic light-detector chip (either a CCD (charge-coupled device) or an alternative technology called a CMOS chip) immediately behind the lens, which converts the light energy into electricity. In a traditional camera, there's no chip; instead, the incoming energy is captured by a piece of plastic that is sensitive to light, better known as the film. The light energy leaves a permanent trace by causing a chemical and physical transformation of the film. (Incidentally, light doesn't just mean visible light: you can, in theory, make a photograph from any kind of incoming light: infrared, ultraviolet X-rays, or whatever you wish.)

Photo: A typical piece of 35mm color photographic film. You can just about make out that there are four "frames" (individual pictures) here, side by side. So a roll of film with 36 frames on it would be about nine times this length. Note the sprocket holes at the top and bottom for winding the film accurately through the camera.

Photo: Taking a photo the old-fashioned way. Photo by J.E. Pasonault courtesy of US Library of Congress.

When you're ready to take a photo, you point the camera at your subject, click a button, and the shutter opens briefly, allowing light rays to pass through the aperture and strike the film, usually for a fraction of a second before it closes up again. The incoming light rays cross over as they enter, with rays from the top of the object ending up on the bottom of the film and vice-versa, thus producing an upside-down (inverted) image on the film.

Film is very sensitive to light: only a tiny amount of light energy is needed to make a photograph and too much light will destroy it. To produce a perfect photo, you have to let exactly the right amount of light hit the film, which is called the exposure. The exposure depends on two factors: how long the shutter is open (the shutter speed) and how widely it's open (the aperture). Shutter speed is measured in seconds (anything from about 1/10,000 second to 30 seconds). Aperture is measured in units called f-stops (or just "stops" for short), such as f/4 and f/8. Smaller f numbers (such as 1 or 2) mean large apertures, so more light gets in; higher f numbers (such as 16, 22, or 32) mean small apertures, so less light is let in.

Automatic, compact, "point-and-shoot" cameras produce a reasonable image with the click of a single button: they use photocells (electronic light sensors) to automatically adjust the shutter speed and aperture and fire out invisible infrared or ultrasound beams to set the focus automatically as well. Although sophisticated professional cameras often have automatic controls, they also allow completely manual operation: before you can take a photo, you have to adjust the focus, set the exposure time, and adjust the size of the aperture. With manual cameras, you have to adjust the exposure time and aperture setting to compensate for one another, because both of them affect the amount of light reaching the film.

Photo: Need to take a photo at long range? Try a telephoto lens like this one, which is 400mm (~15 inches) long! Photo by Angela M. Virnig courtesy of US Navy and Wikimedia Commons.

Other features of film cameras

Most film cameras also have a viewfinder (so you can see how your photograph will appear), a xenon flash lamp (which adds enough extra light energy to activate the film, even in dark conditions), and self-timer mechanism (so you can photograph yourself without anyone's help). Inexpensive cameras generally have a viewfinder mounted to one side and above the main lens, so the image you compose is only an approximation of what you'll see on the final photograph. Professional cameras use a system called SLR (single lens reflex), in which prisms and mirrors allow you to look through the actual lens of the camera and see an exact replica of the photo you'll take. Most digital cameras (even inexpensive ones) produce a faithful copy of the final image, like SLRs, because the image you see on the LCD screen at the back is produced by the CCD or CMOS chip that captures the final photograph.

While inexpensive point-and-shoot cameras generally have fixed lenses, professional SLRs are designed so you can unscrew one lens and screw in longer or shorter ones, as necessary, according to what you want to photograph. Lenses generally range from about 20mm (~0.8 inch) wide-angle lenses (for photographing something relatively wide and quite close) to 800mm (30 inches) or so ("elephant gun", telephoto lenses for taking very distant shots of quite narrowly defined areas). In between these two extremes, a typical everyday lens is about 50mm (2 inches) long. Many modern cameras have zoom lenses, usually powered by small electric motors, that can be moved back and forth between two fixed points to provide a whole range of different magnifications.

Artwork: How an SLR camera works: 1) Light enters at the front and passes through the lenses (2) and iris diaphragm (3), which has metal blades that open and close to let in more or less light. Inside the camera, the light bounces off a hinged mirror (4) and shoots up into a penta-prism (5, five-sided prism), which bounces it into the viewfinder (6) and your eye. When you press the shutter-release button, the mirror (4) flips down out of the way. The light from the lens (dotted line) now passes straight through to the back of the camera, through the shutter (7), and hits the film instead (8). This type of design ensures that the image you see through the viewfinder is exactly like the image captured on the film.

How does photographic film work?

Photo: A photographic darkroom is usually lit with dim green or red light to prevent damage to undeveloped film. Photo by Leah Stiles courtesy of US Navy and Wikimedia Commons.

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.

Developing and printing 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.

Artwork: The photographic process captures an image as a photograph in three main steps. 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.

First, the film is dipped in an alkaline 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.

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.

Types of film

Photo: Kodak dominated the photographic film market throughout the 20th century, but its revolutionary product—plastic photographic film—has now largely been replaced by instant digital photography. Photographs in Carol M. Highsmith's America Project in the Carol M. Highsmith Archive, courtesy of US Library of Congress.

Most film is sold in light-tight cartridges that you snap into your camera. Inside, the cartridges contain a long reel of plastic film separated into one, two, or three dozen rectangular frames that measure 24mm x 36mm (this standard size is called 35mm film). The top and bottom of the reel is punched with little holes so each section of the film can be wound out of the way after a photo is taken, releasing an unexposed frame ready for the next photo. (Cameras generally have either a spool mechanism slowly wound by hand or automatically and very quickly wound by an electric motor.)

There are numerous different kinds of film designed for taking different kinds of photo. Black and white film is sensitive only to the presence or absence of light, so it shows images only as shades of gray. Color film effectively works the same way as black and white only with three separate layers, one sensitive to blue, one sensitive to green, and one sensitive to red light. Films are also designed to work in widely different light conditions: generally speaking, you need to use a fast film (one that forms an image with relatively little light) in dark, indoor conditions and a slow film (one that needs more light) in bright, outdoor conditions. The film speed is indicated by a number called the ISO rating: ISO numbers of 100 are slow, 400 or more are fast, and 200 are good for general-purpose photography. Fast films generally produce grainier, more blurry images than slow ones so, as a rule, photographers never use a faster film than is absolutely necessary. Modern cameras automatically detect film speed using a system called DX coding, which simply involves the camera reading a barcode printed on the film container. For no real reason other than convention, good digital cameras also tend to use ISO ratings to indicate how quickly photographs are taken in different light conditions.

A brief history of photography

Who invented cameras? Here's a brief history of the key moments in the development of film-based photography:

• Ancient times: People figure out how to produce images on the walls of darkened rooms by making holes in the shutters or drapes on an opposite wall. This is called a camera obscura.
  

  Artwork: One of the earliest drawings showing the optics of a pinhole camera, by Gemma Frisius, a Dutch physician and scientific instrument maker, dating from 1544. The sun (right) streams through the pinhole (red) and makes an inverted image on the wall of the room (left).

• 1727: German physicist Johann Schulze discovers the essential piece of science that will underpin all photography until the late 20th century: silver-based chemical salts are sensitive to light.
• Late 18th century: Sir Humphry Davy and Thomas Wedgwood use silver-coated paper to make crude copies of paintings.
• 1827: Frenchman Joseph Nicéphore Niépce makes the first photograph, a view from his window, using a metal plate coated with silver chemicals exposed to the light for eight-hours.
• 1831: With Niépce's help, another Frenchman, Louis Daguerre (pictured, right), perfects a method of making extremely detailed photographic images on silver plates, named daguerreotypes. The method becomes hugely popular in the United States.
• 1838: Daguerre takes the first known photograph of a person at the Boulevard du Temple, Paris. Photo of Louis Daguerre courtesy of US Library of Congress.
  
• 1839: Henry William Fox Talbot figures out how to make photographs on light-sensitive paper coated with silver-based chemicals. He invents the negative-positive process and the way of making multiple prints from a single negative. His pictures are initially called Talbotypes or Calotypes until a friend of his, English astronomer Sir John Herschel, suggests "photographs" might be a better name.
• 1851: English artist Frederick Scott Archer invents a new way of making photographs using wet glass plates. Although cheaper than daguerreotypes and better quality than Fox Talbot's method, wet plates are cumbersome and have to be developed immediately after they are exposed.
• 1859: Celebrated British physicist James Clerk Maxwell produces the first color photograph (of a tartan ribbon).
• 1871: Englishman Richard Maddox improves on Frederick Scott Archer's work by figuring out how to make photographs using dry plates and a gelatin emulsion. The method is quicker and allows photographs to be developed some time after exposure (instead of immediately).
• 1883: American George Eastman revolutionizes photography by inventing inexpensive, plastic photographic film.
• 1889: George Eastman makes it possible for millions of people to take up photography by launching his simple, inexpensive Kodak camera with the marketing line: "You Press the Button, We Do the Rest."
• 1896: German Wilhelm Röntgen takes the first X ray photograph (of his wife's hand).
• 1924: The German Leica company begins selling the 35mm photographic film that soon becomes a worldwide standard.
• 1931: Harold Edgerton invents the xenon-strobe flash lamp, making possible photography of objects moving at high speed.
• 1930s: Inexpensive Kodak and Agfa color film becomes available.
• 1947: American physicist Edwin Land invents a camera that can produce instant photographs, named the Polaroid Land Camera.
  

  Photo: Edwin Land revolutionized photography with his Polaroid cameras, which allowed people to see their finished photographs within a minute of snapping them. Photo courtesy of Ralf Steinberger published on Wikimedia Commons under a Creative Commons Licence.

• 1963: Edwin Land releases an improved, color version of his Polaroid camera.
• 1985: Cameras appear with DX coding, a barcode system that allows the camera to read the film speed (and number of frames) automatically from the film container and adjust itself automatically.
• 1990s: Inexpensive digital cameras begin to make film obsolete. Some photographers stick with the old technology, but many stores stop selling film altogether.
• 2008: The Polaroid company stops manufacturing instant film due to lack of demand. The following year, a group of Dutch enthusiasts called The Impossible Project (later renamed Polaroid Originals) begin making their own replacement film.
• 2010: Instagram, the photo-sharing app, is founded by Kevin Systrom and Mike Krieger, originally targeted at iPhone users.
• 2012: The Eastman Kodak company, which pioneered film photography, files for bankruptcy after failing to meet the challenge from digital cameras.

Photo: Right: If you'd been a keen amateur photographer in the 1930s, you might have used a compact camera like this Soho Cadet (made in London, England), which packs into a sturdy outer case made from tough Bakelite plastic. The front of the case hinges down to reveal the camera mechanism inside. You pull on the lens and it extends out from the case on a leather bellows. You can focus the camera very crudely by pulling the bellows out from the case slightly more or slightly less, so adjusting the distance between the lens and the film. You can also crudely adjust the exposure with a slider control just above the lens. You can just make out the little viewfinder on the top right of the lens (as we look at it).

You might like these other articles on our site covering similar topics:

• CCDs (charge-coupled devices)
• Digital cameras
• Light

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