Photography is undoubtedly one of the most important
inventions in history -- it has truly transformed how people
conceive of the world. Now we can "see" all sorts of things
that are actually many miles -- and years -- away from us.
Photography lets us capture moments in time and preserve them
for years to come.
A fully manual single-lens-reflex
The basic technology that makes all of this possible is
fairly simple. A still film camera is made of three basic
elements: an optical element (the lens), a chemical element
(the film) and a mechanical element (the camera body itself).
As we'll see, the only trick to photography is calibrating and
combining these elements in such a way that they record a
crisp, recognizable image.
There are many different ways of bringing everything
together. In this edition of HowStuffWorks,
we'll look at a manual single-lens-reflex (SLR) camera.
This is a camera where the photographer sees exactly the same
image that is exposed to the film and can adjust everything by
turning dials and clicking buttons. Since it doesn't need any
electricity to take a picture, a manual SLR camera provides an
excellent illustration of the fundamental processes of
Bending Light The optical component of the
camera is the lens. At its simplest, a lens is just
a curved piece of glass or plastic. Its job is to take the
beams of light bouncing off of an object and redirect them so
they come together to form a real image -- an image
that looks just like the scene in front of the lens.
But how can a piece of glass do this? The process is
actually very simple. As light
travels from one medium to another, it changes speed. Light
travels more quickly through air than it does through glass,
so a lens slows it down.
When light waves enter a piece of glass at an angle, one
part of the wave will reach the glass before another and so
will start slowing down first. This is something like pushing
a shopping cart from pavement to grass, at an angle. The right
wheel hits the grass first and so slows down while the left
wheel is still on the pavement. Because the left wheel is
briefly moving more quickly than the right wheel, the shopping
cart turns to the right as it moves onto the grass.
The effect on light is the same -- as it enters the glass
at an angle, it bends in one direction. It bends again
when it exits the glass because parts of the light wave enter
the air and speed up before other parts of the wave. In a
standard converging, or convex lens, one or both
sides of the glass curves out. This means rays of light
passing through will bend toward the center of the lens on
entry. In a double convex lens, such as a magnifying
glass, the light will bend when it exits as well as when it
This effectively reverses the path of light from an object.
A light source -- say a candle -- emits light in all
directions. The rays of light all start at the same point --
the candle's flame -- and then are constantly diverging. A
converging lens takes those rays and redirects them so they
are all converging back to one point. At the point where the
rays converge, you get a real image of the candle. In the next
couple of sections, we'll look at some of the variables that
determine how this real image is formed.
Focus We saw in the last section that a
real image is formed by light moving through a convex
lens. The nature of this real image varies depending on how
travels through the lens. This light path depends on two major
The angle of the light beam's entry into the lens
The structure of the lens
The angle of light
entry changes when you move the object closer or farther
away from the lens. You can see this in the diagram below. The
light beams from the pencil point enter the lens at a sharper
angle when the pencil is closer to the lens and a more obtuse
angle when the pencil is farther away. But overall, the lens
only bends the light beam to a certain total degree, no matter
how it enters. Consequently, light beams that enter at a
sharper angle will exit at a more obtuse angle, and vice
versa. The total "bending angle" at any particular point on
the lens remains constant.
As you can see, light beams from a closer point converge
farther away from the lens than light beams from a point
that's farther away. In other words, the real image of a
closer object forms farther away from the lens than the real
image from a more distant object.
You can observe this phenomenon with a simple experiment.
Light a candle in the dark, and hold a magnifying glass
between it and the wall. You will see an upside down image of
the candle on the wall. If the real image of the candle does
not fall directly on the wall, it will appear somewhat blurry.
The light beams from a particular point don't quite converge
at this point. To focus the image, move the magnifying glass
closer or farther away from the candle.
This is what you're doing when you turn the lens of a
camera to focus it -- you're moving it closer or farther away
from the film surface.
As you move the lens, you can line up the focused real image
of an object so it falls directly on the film surface.
Lens Shape and Image Size In the last
section, we saw that at any one point, a lens bends light
beams to a certain total degree, no matter the light beam's
angle of entry. This total "bending angle" is determined by
the structure of the lens.
Lenses in the
A camera lens is
actually several lenses combined into one unit. A single
converging lens could form a real image on the film, but
it would be warped by a number of aberrations.
One of the most significant warping factors is that
different colors of light bend differently when moving
through a lens. This chromatic aberration
essentially produces an image where the colors are not
lined up correctly.
Cameras compensate for this using several lenses made
of different materials. The lenses each handle colors
differently, and when you combine them in a certain way,
the colors are realigned.
In a zoom lens, you can move different lens
elements back and forth. By changing the distance
between particular lenses, you can adjust the
magnification power -- the focal length -- of the lens
as a whole.
A lens with a rounder shape (a center that extends out
farther) will have a more acute bending angle. Basically,
curving the lens out increases the distance between different
points on the lens. This increases the amount of time that one
part of the light wave is moving faster than another part, so
the light makes a sharper turn.
Increasing the bending angle has an obvious effect. Light
beams from a particular point will converge at a point closer
to the lens. In a lens with a flatter shape, light beams will
not turn as sharply. Consequently, the light beams will
converge farther away from the lens. To put it another way,
the focused real image forms farther away from the lens when
the lens has a flatter surface.
Increasing the distance between the lens and the real image
actually increases the total size of the real image. If you
think about it, this makes perfect sense. Think of a
projector: As you move the projector farther away from the
screen, the image becomes larger. To put it simply, the light
beams keep spreading apart as they travel toward the screen.
The same basic thing happens in a camera. As the distance
between the lens and the real image increases, the light beams
spread out more, forming a larger real image. But the size of
the film stays constant. When you attach a very flat lens, it
projects a large real image but the film is only exposed to
the middle part of it. Basically, the lens zeroes in on the
middle of the frame, magnifying a small section of the scene
in front of you. A rounder lens produces a smaller real image,
so the film surface sees a much wider area of the scene (at
Professional cameras let you attach different lenses so you
can see the scene at various magnifications. The magnification
power of a lens is described by its focal length. In
cameras, the focal length is defined as the distance between
the lens and the real image of an object in the far distance
(the moon for example). A higher focal length number indicates
a greater image magnification.
A standard 50 mm lens doesn't significantly
shrink or magnify the
Different lenses are suited to different situations. If
you're taking a picture of a mountain range, you might want to
use a telephoto lens, a lens with an especially long
focal length. This lens lets you zero in on specific elements
in the distance, so you can create tighter compositions. If
you're taking a close-up portrait, you might use a
wide-angle lens. This lens has a much shorter focal
length, so it shrinks the scene in front of you. The entire
face is exposed to the film even if the subject is only a foot
away from the camera. A standard 50 mm camera lens doesn't
significantly magnify or shrink the image, making it ideal for
shooting objects that aren't especially close or far away.
Recording Light The chemical component in a
traditional camera is film. Essentially, when you
expose film to a real image, it makes a chemical record
of the pattern of light.
What's in a
As it turns out,
the term photography describes the photographic process
quite accurately. Sir John Herschel, a 19th century
astronomer and one of the first photographers, came up
with the term in 1839. The term is a combination of two
Greek words -- photos meaning light and
graphein meaning writing (or drawing).
The term camera comes from camera obscura,
Latin for "dark room." The camera obscura was actually
invented hundreds of years before photography. A
traditional camera obscura was a dark room with light
shining through a lens or tiny hole in the wall. Light
passed through the hole, forming an upside-down real
image on the opposite wall. This effect was very popular
with artists, scientists and curious
does this with a collection of tiny light-sensitive grains,
spread out in a chemical suspension on a strip of plastic.
When exposed to light, the grains undergo a chemical reaction.
Once the roll is finished, the film is developed -- it is
exposed to other chemicals, which react with the
light-sensitive grains. In black and white film, the developer
chemicals darken the grains that were exposed to light. This
produces a negative, where lighter areas appear darker and
darker areas appear lighter, which is then converted into a
positive image in printing.
Color film has three different layers of light-sensitive
materials, which respond, in turn, to red, green and blue.
When the film is developed, these layers are exposed to
chemicals that dye the layers of film. When you overlay the
color information from all three layers, you get a full-color
Snap Shot In the last two sections we looked
at the basic idea of photography -- you create a real image
with a converging lens, and you record the light pattern of
this real image on a layer of light-sensitive material.
Conceptually, this is all that's involved in taking a picture.
But to capture a clear image, you have to carefully control
how everything comes together.
Obviously, if you were to lay a piece of film on the ground
and focus a real image onto it with a converging lens, you
wouldn't get any kind of usable picture. Out in the open,
every grain in the film would be completely exposed to light.
And without any contrasting unexposed areas, there's no
To capture an image, you have to keep the film in complete
darkness until it's time to take the picture. Then, when you
want to record an image, you let some light in. At its most
basic level, this is all the body of a camera is -- a sealed
box with a shutter that opens and closes between the
lens and film. In fact, the term camera is shortened from
camera obscura, literally "dark room" in Latin.
For the picture to come out right, you have to precisely
control how much light hits the film. If you let too much
light in, too many grains will react, and the picture will
appear washed out. If you don't let enough light hit the film,
too few grains will react, and the picture will be too dark.
In the next section, we'll look at the different camera
mechanisms that let you adjust the exposure.
The Right Light In the last section, we saw
that you need to carefully control the film's exposure to
light, or your picture will come out too dark or too bright.
So how do you adjust this exposure level? You have to consider
two major factors:
How much light is passing through the lens
How long the film is exposed
To increase or decrease the amount of light passing through
the lens, you have to change the size of the aperture
-- the lens opening. This is the job of the iris
diaphragm, a series of overlapping metal plates that can
fold in on each other or expand out. Essentially, this
mechanism works the same way as the iris in your eye -- it
opens or closes in a circle, to shrink or expand the diameter
of the lens. When the lens is smaller, it captures less light,
and when it is larger, it captures more light.
The plates in the iris diaphragm fold in on
each other to shrink the aperture and expand out to make
The length of exposure is determined by the shutter
speed. Most SLR cameras use a focal plane shutter.
This mechanism is very simple -- it basically consists of two
"curtains" between the lens and the film. Before you take a
picture, the first curtain is closed, so the film won't be
exposed to light. When you take the picture, this curtain
slides open. After a certain amount of time, the second
curtain slides in from the other side, to stop the exposure.
When you click the camera's
shutter release, the first curtain slides open, exposing the
film. After a certain amount of time, the second shutter
slides closed, ending the exposure. The time delay is
controlled by the camera's shutter speed
This simple action is controlled by a complex mass of
gears, switches and springs, like you might find inside a
watch. When you hit the shutter button, it releases a
lever, which sets several gears in motion. You can tighten or
loosen some of the springs by turning the shutter speed knob.
This adjusts the gear mechanism, increasing or decreasing the
delay between the first curtain opening and the second curtain
closing. When you set the knob to a very slow shutter speed,
the shutter is open for a very long time. When you set the
knob to a very high speed, the second curtain follows directly
behind the first curtain, so only a tiny slit of the film
frame is exposed at any one time.
The ideal exposure depends on the size of the
light-sensitive grains in the film. A larger grain is more
likely to absorb light photons than a smaller grain. The size
of the grains is indicated by a film's speed, which is
printed on the canister. Different film speeds are suited to
different types of photography -- 100 ISO film, for example,
is optimal for shots in bright sunlight, while 1600 film
should only be used in relatively low light.
Inside a manual SLR
camera, you'll find an intricate puzzle of gears and
springs. Click on each picture for a high-resolution
As you can see, there's a lot involved in getting the
exposure right -- you have to balance film speed, aperture
size and shutter speed to fit the light level in your shot.
Manual SLR cameras have a built-in light meter to help you do
this. The main component of the light meter is a panel of
semi-conductor light sensors that are sensitive to light
energy. These sensors express this light energy as electrical
energy, which the light meter system interprets based on the
film and shutter speed.
In the next section, we'll see how an SLR camera body
directs the real image to the viewfinder before you take the
shot, and then directs it to the film when you press the
SLR vs. Point-and-Shoot There are two types
of consumer film cameras on the market -- SLR cameras and
"point-and-shoot" cameras. The main difference is how the
photographer sees the scene. In a point-and-shoot camera, the
viewfinder is a simple window through the body of the camera.
You don't see the real image formed by the camera lens, but
you get a rough idea of what is in view.
In an SLR camera, you see the actual real image that the
film will see. If you take the lens off of an SLR camera and
look inside, you'll see how this works. The camera has a
slanted mirror positioned between the shutter and the lens,
with a piece of translucent glass and a prism positioned above
it. This configuration works like a periscope -- the real
image bounces off the lower mirror on to the translucent
glass, which serves as a projection screen. The prism's job is
to flip the image on the screen, so it appears right side up
again, and redirect it on to the viewfinder window.
When you click the shutter button, the camera quickly
switches the mirror out of the way, so the image is directed
at the exposed film. The mirror is connected to the shutter
timer system, so it stays open as long as the shutter is open.
This is why the viewfinder is suddenly blacked out when you
take a picture.
The mirror in an SLR camera directs the real
image to the viewfinder. When you hit the shutter
button, the mirror flips up so the real image is
projected onto the
In this sort of camera, the mirror and the translucent
screen are set up so they present the real image exactly as it
will appear on the film. The advantage of this design is that
you can adjust the focus and compose the scene so you get
exactly the picture you want. For this reason, professional
photographers typically use SLR cameras.
These days, most SLR cameras are built with both manual and
automatic controls, and most point and shoot cameras are fully
automatic. Conceptually, automatic cameras are pretty much the
same as fully manual models, but everything is controlled by a
central microprocessor instead of the user. The central
microprocessor receives information from the autofocus
system and the light meter. Then it activates several
small motors, which adjust the lens and open and close the
aperture. In modern cameras, this a pretty advanced computer
Automatic point-and-shoot camera use circuit
boards and electric motors, instead of gears and
In the next section, we'll look at the other end of the
spectrum -- a camera design with no complex machinery, no lens
and barely any moving parts.
Homemade Cameras As we've seen in this
article, even the most basic, completely manual SLR is a
complex, intricate machine. But cameras are not inherently
complex -- in fact, the basic elements are so simple you can
make one yourself with only a few inexpensive supplies.
The simplest sort of homemade camera doesn't use a lens to
create a real image -- it gathers light with a tiny hole.
cameras are easy to make and a lot of fun to use -- the
only hard part is that you have to develop the film yourself.
A pinhole camera is simply a box with a tiny hole in one
side and some film or photographic paper on the opposite size.
If the box is otherwise "light-tight," the light coming
through the pinhole will form a real image on the film. The
scientific principle behind this is very simple.
If you were to shine a flashlight in a dark room, through a
tiny hole in a wide piece of cardboard, the light would form a
dot on the opposite wall. If you moved the flashlight, the
light dot would also move -- light beams from the flashlight
move through the hole in a straight line.
In a larger visual scene, every particular visible point
acts like this flashlight. Light reflects off each point of an
object and travels out in all directions. A small pinhole lets
in a narrow beam from each point in a scene. The beams travel
in a straight line, so light beams from the bottom of the
scene hit the top of the piece of film, and vice-versa. In
this way, an upside down image of the scene forms on the
opposite side of the box. Since the hole is so small, you need
a fairly long exposure time to let enough light in.
There are a number of ways to build this sort of camera --
some enthusiasts have even used old refrigerators and cars as
light-tight boxes. One of the most popular designs uses an
ordinary cylinder oatmeal box, coffee can, or similar
container. Its easiest to use a cardboard container with a
removable plastic lid.
You can build this camera in a few simple steps:
The first thing to do is paint the lid black, inside
and out. This helps light-proof the box. Be sure to use
flat black paint, rather than glossy paint that will
reflect more light.
Cut a small hole (about the size of a matchbox) in
the center of the canister bottom (the nonremovable
Cut out a piece of heavy-duty aluminum foil, or
heavy black paper, about twice the size of the hole in the
bottom of the canister.
Take a No. 10 sewing needle and carefully make a hole
in the center of the foil. You should only insert the
needle halfway, or the hole will be too big. For best
results, position the foil between two index cards and
rotate the needle as you push it through.
Tape the foil over the hole in the bottom of the
canister, so the pinhole is centered. Attach the foil
securely, with black tape, so light only shines through
All you need for the shutter is a piece of heavy
black paper large enough to cover most of the cannister
bottom. Tape one side of the paper securely to the side
of the cannister bottom, so it makes a flap over the
pinhole in the middle. Tape the other side of the flap
closed on the other side of the pinhole. Keep the flap
closed until you are ready to take a picture.
To load the camera, attach any sort of film or
photographic paper to the inside of the canister lid. Of
course, for the film to work, you must load it and develop
it in complete darkness. With this camera design, you won't
be able to simply drop the film off at the drug store --
you'll have to develop it yourself or get someone to help
Choosing a good camera design, film type and
exposure time is largely a matter of trial and error. But, as
any pinhole enthusiast will tell you, this experimentation is
the most interesting thing about making your own camera. To
find out more about pinhole photography and see some great
camera designs, check out some of the sites listed on the next
Throughout the history of photography, there have been
hundreds of different camera systems. But amazingly, all these
designs -- from the simplest homemade box camera to the newest
digital camera -- combine the same basic elements: a lens
system to create the real image, a light-sensitive sensor to
record the real image, and a mechanical system to control how
the real image is exposed to the sensor. And when you get down
to it, that's all there is to photography!
For more information, check out the links on the next page.