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 SLR (single-lens reflex) film camera dating from the mid-1990s. This one's made by Nikon and includes a motorized film-winding unit (on the left, where the photographer's finger is pressing). Older cameras had mechanical winding levers or knobs to advance the film after each shot. Photo courtesy of NASA and Internet Archive.
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,
ultravioletX-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.
How does a film camera work?
Before digital cameras came along, photography involved capturing light rays on silver-based film, as shown in the picture below. Light entered the front of the camera (yellow) through the aperture and lens (red) and hit a piece of film (green) wound out from the spools at the back (brown). Originally, photography was a very specialized and skilful business— until this little invention came along and allowed anyone and everyone to take photos. It's George Eastman's original Kodak camera.
Artwork: Illustration from "US Patent #388,850: Camera" granted on September 4, 1888, courtesy of US Patent and Trademark Office with our coloring and labeling added for clarity.
You can't easily open up a digital camera to see how it works, and even if you do, you don't learn that much. But film cameras reveal their secrets more freely. Open the back and you can see the spools (left and right) where the film goes. In the middle, you can see the back of the shutter mechanism and the lens in front of it. The viewfinder is the little clear
plastic window at the top:
Photo: Inside a typical film camera.
The key features of a film camera are:
A plastic or metal case that is completely light-tight to protect the light-sensitive film.
An aperture: a small circular hole that lets light into the case for the short period when
you want to take a photo.
The aperture is at the center of a bigger structure called an iris or iris diaphragm—a
set of overlapping blades, like the ones you see at the start of a James Bond film,
that can open or close to to let more or less light in through the aperture.
A shutter mechanism: a spring-loaded
door that opens for a precise amount of time before closing up again to let light through
One or more lenses in front of the shutter. The
lenses are a crucial
part of the camera and do several jobs at once. First, they scale
down the large, incoming image of the world so it fits into a much
smaller area of film: no-one really wants life-sized photos! Second,
lenses concentrate the incoming light energy so the image forms on
the film more quickly and the camera can be used in darker
conditions than would otherwise be the case. Third, they bring the
light rays into a sharp focus exactly on the surface of the film, so
you get a clear, sharp, image rather than a blurred, fuzzy
impression. Finally, they also minimize the distance between the
aperture and the film so cameras can be made relatively small and
portable. When you adjust the focus on a camera, you're actually
moving one or more of the lenses back and forth (closer to or
further from the object and the film) to make different parts of a
scene appear sharper on the film, according to whether you want to
emphasize near or distant objects in your photograph.
A roll or piece of film (on the back wall of the camera directly
opposite the shutter).
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
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
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
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.
Photo: Left: A photographic color negative of some
recycling dumpsters looks like this. Right: When it's printed,
the colors are reversed and come out looking as you'd expect. I've simulated the effect of printing by reversing the
colors digitally with a computer graphics package. Notice how the real-life red dumpster (in the center in the positive image
on the right) turns green in the negative, while the yellow one (on the right in the positive image) turns blue in the negative.
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
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
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.
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
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.
1963: Edwin Land releases an improved, color version of his Polaroid
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).
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