have been using camera and
film for more than 100 years, both for still photography and
movies. There is something magical about the process -- humans
are visual creatures, and a picture really does paint a
thousand words for us!
Despite its long history, film remains the best way to
capture still and moving pictures because of its incredible
ability to record detail in a very stable form. In this
edition of HowStuffWorks,
you'll learn all about how film works, both inside your camera
and when it is developed, so you can understand exactly what
is going on!
What does it really mean when you
"take" a picture with a camera?
When you click the shutter, you have frozen a moment in time
by recording the visible light
reflected from the objects in the camera's field of view. In
order to do that, the reflected light causes a chemical change
to the photographic film inside the camera. The chemical
record is very stable, and can be subsequently developed,
amplified and modified to produce a representation (a print)
of that moment that you can put in your photo album or your
wallet, or that can be reproduced millions of times in
magazines, books and newspapers.
You can even scan the
photograph and put it on a Web site.
To understand the whole process, you'll learn some of the
science behind photography -- exposing the image, processing
the image, and producing a print of the image. It all starts
with an understanding of the portion of the electromagnetic
spectrum that human eyes are
sensitive to: light.
Light and Energy
Energy from the sun comes to
the Earth in visible and invisible portions of the
electromagnetic spectrum. Human eyes are
sensitive to a small portion of that spectrum that includes
the visible colors -- from the longest visible
wavelengths of light (red) to the shortest wavelengths (blue).
Microwaves, radio waves, infrared, and ultraviolet waves
are portions of the invisible electromagnetic spectrum. We
cannot see these portions of the spectrum with our eyes, but
we have invented devices (radios, infrared
detectors, ultraviolet dyes, etc.) that let us detect
these portions as well.
Light is neither a wave nor a particle, but has properties
of both. Light can be focused like a wave, but its energy is
distributed in discrete packets called photons. The
energy of each photon is inversely related to the wavelength
of the light -- blue light is the most energetic, while red
light has the least energy per photon of exposure. Ultraviolet
light (UV) is more energetic, but invisible to human eyes.
Infrared light is also invisible, but if it is strong enough
our skin detects it as heat.
It is the energy in each photon of light that causes a
chemical change to the photographic detectors that are coated
on the film. The process whereby electromagnetic energy causes
chemical changes to matter is known as photochemistry.
By carefully engineering materials, they can be chemically
stable until they are exposed to radiation (light).
Photochemistry comes in many different forms. For example,
specially formulated plastics can be hardened (cured) by
exposure to ultraviolet light, but exposure to visible light
has no effect. When you get a sun tan,
a photochemical reaction has caused the pigments in your skin
to darken. Ultraviolet rays are particularly harmful to your
skin because they are so energetic.
Inside a Roll of Film
If you were to open a
35-mm cartridge of color print film, you would find a long
strip of plastic that has coatings on each side. The heart of
the film is called the base, and it starts as a
transparent plastic material (celluloid) that is 4 thousandths
to 7 thousandths of an inch (0.025 mm) thick. The back side of
the film (usually shiny) has various coatings that are
important to the physical handling of the film in manufacture
and in processing.
It is the other side of the film that we are most
interested in, because this is where the photochemistry
happens. There may be 20 or more individual layers coated here
that are collectively less than one thousandth of an inch
thick. The majority of this thickness is taken up by a very
special binder that holds the imaging components together. It
is a marvelous, and ubiquitous material called gelatin.
A specially purified version of edible
gelatin is used for photography -- yes, the same thing
that makes Jell-O jiggly holds film together, and has done so
for more than 100 years! Gelatin comes from animal hides and
bones. Thus, there is an important link between a cow, a
hamburger and a roll of film that you might not have
Some of the layers coated on the transparent film do not
form images. They are there to filter light, or to control the
chemical reactions in the processing steps. The imaging layers
contain sub-micron sized grains of silver-halide
crystals that act as the photon detectors. These crystals
are the heart of photographic film. They undergo a
photochemical reaction when they are exposed to various forms
of electromagnetic radiation -- light. In addition to visible
light, the silver-halide grains can be sensitized to infrared
Silver-halide grains are manufactured by combining
silver-nitrate and halide salts (chloride, bromide and iodide)
in complex ways that result in a range of crystal sizes,
shapes and compositions. These primitive grains are then
chemically modified on their surface to increase their light
The unmodified grains are only sensitive to the blue
portion of the spectrum, and they are not very useful in
camera film. Organic molecules known as spectral
sensitizers are added to the surface of the grains to make
them more sensitive to blue, green and red light. These
molecules must adsorb (attach) to the grain surface and
transfer the energy from a red, green, or blue photon to the
silver-halide crystal as a photo-electron. Other chemicals are
added internally to the grain during its growth process, or on
the surface of the grain. These chemicals affect the light
sensitivity of the grain, also known as its photographic
speed (ISO or ASA rating).
When you purchase a roll of film for your camera, you have
a lot of choices. Those products that have the word "color" in
their name are generally used to produce color prints that you
can hold in your hand and view by reflected light. The
negatives that are returned with your prints are the exposures
that were made in your camera. Those products that have the
word "chrome" in their name produce a color transparency
(slides) that requires some form of projector for viewing. In
this case, the returned slides are the actual film that was
exposed in your camera.
Film comes with
an ASA (American Standards Association) or ISO
(International Standards Organization) rating that tells
you its speed. The ISO and ASA scales are
identical. Here are some of the most common film speeds:
- ISO 100 - good for outdoor photography in bright
- ISO 200 - good for outdoor photography or brightly
lit indoor photography
- ISO 400 - good for indoor photography
- ISO 1000 or 1600 - good for indoor photography
where you want to avoid using a flash
Once you decide on prints or slides, the next major
decision is the film speed. Generally, the relative
speed rating of the film is part of its name (MYColor Film
200, for example). ISO and ASA speed ratings are also
generally printed somewhere on the box. The higher the number,
the "faster" the film. "Faster" means increased light
sensitivity. You want a faster film when you're photographing
quickly moving objects and you want them to be in focus, or
when you want to take a picture in dimly lit surroundings
without the benefit of additional illumination (such as a
When you make film faster, the trade-off is that the
increased light sensitivity comes from the use of larger
silver-halide grains. These larger grains can result in a
blotchy or "grainy" appearance to the picture, especially if
you plan to make enlargements from a 35-mm negative.
Professional photographers may use a larger-format negative to
reduce the degree of enlargement and the appearance of grain
in their prints. The trade-off between photographic speed and
graininess is an inherent part of conventional photography.
Photographic-film manufacturers are constantly making
improvements that result in faster films with less grain.
A slow-speed film is desirable for portrait photography,
where you can control the lighting of the subject, the subject
is stationary, and you are likely to want a large print from
the negative. The finer silver-halide grains in such film
produce the best results.
The advanced amateur photographer might encounter
additional film designations such as tungsten balanced
or daylight balanced. A tungsten-balanced film is meant
to be used indoors where the primary source of light is from
tungsten filament light
bulbs. Since the visible illumination coming from a light
bulb is different than from the sun (daylight), the spectral
sensitivity of the film must be modified to produce a pleasing
picture. This is most important when using a transparency
Taking a Picture
The first step after
loading the film is to focus the image on the surface
of film. This is done by adjusting glass or plastic lenses
that bend the reflected light from the objects onto the film.
Older cameras required manual adjustment, but today's modern
cameras use solid-state detectors to automatically focus the
image, or else they are fixed-focus (no adjustment possible).
Next, the proper exposure must be set. The film
speed is the first factor, and most of today's cameras
automatically sense which speed film is being used from the
markings that are on the outside of a 35-mm cartridge. The
next two factors are interdependent, since the exposure to the
film is the product of light intensity and exposure
time. The light intensity is determined by how much
reflected light is reaching the film plane. You used to have
to carry a light meter to set the camera exposure, but most of
today's cameras have built-in exposure meters. In addition to
the brightness of the scene, the larger the diameter of the
camera lens, the more light will be gathered. Obviously, the
trade-off here is the cost of the camera and the resulting
size and weight. If there is too much light reaching the film
plane for the exposure-time setting, the lens can be "stepped
down" (reduced in diameter) using the f-stop
adjustment. This is just like the iris in your eye reacting to
Photographic film has a limited exposure latitude.
If it is underexposed, it will not detect all the reflected
light from a scene. The resulting print appears to be muddy
black and lacks detail. If it is over-exposed, all of the
silver-halide grains are exposed so there is no discrimination
between lighter and darker portions of the scene. The print
appears to be washed out, with little color intensity.
There is an advantage to having a faster film in your
camera. It allows you to have a smaller aperture
setting for the same exposure time. This smaller aperture
diameter produces a larger depth of field. Depth of
field determines how much of the subject matter in your
print is in focus. Sometimes, you may want to have a limited
depth of field, so only the primary object is in focus and the
background is out of focus.
So, either manually or automatically, you now have an image
that is focused on the film surface, and the proper exposure
has been set through a combination of film speed, aperture
settings (f-stop) and exposure time (usually fractions of a
second, from one thirtieth to one one-thousandth of a second).
Say cheese and push the button. What happened? While outwardly
unexciting, the moment of exposure is when a lot of
By opening the camera's shutter for a fraction of a second,
you formed a latent image of the visible energy
reflected off the objects in your viewfinder. The brightest
portion of your picture exposed the majority of the
silver-halide grains in that particular part of the film. In
other parts of the image, less light energy reached the film,
and fewer grains were exposed.
When a photon of light is absorbed by the spectral
sensitizer sitting on the surface of a silver-halide grain,
the energy of an electron is raised into the conduction
band from the valence band, where it can be transferred to
the conduction band of the silver-halide-grain electronic
structure. A conduction-band electron can then go on to
combine with a positive hole in the silver-halide lattice and
form a single atom of silver. This single atom of silver is
unstable. However, if enough photoelectrons are present at the
same time in the crystal lattice, they may combine with enough
positive holes to form a stable latent-image site. It
is generally felt that a stable latent-image site is at least
two to four silver atoms per grain. A silver-halide grain
contains billions of silver-halide molecules, and it only
takes two to four atoms of uncombined silver to form the
In color film, this process happens separately for
exposure to the red, green and blue portions of the reflected
light. There is a separate layer in the film for each color:
Red light forms a latent image in the red-sensitive layer of
the film; green light forms a latent image in the
green-sensitive layer; blue light forms a latent image in the
blue-sensitive layer. The image is called "latent" because you
can't detect its presence until the film is processed. The
true photoefficiency of a film is measured by its performance
as a photon detector. Any photon that reaches the film
but does not form a latent image is lost information. Modern
color films generally take from 20 to 60 photons per grain to
produce a developable latent image.
Developing the Film
When you deliver a roll
of exposed film to the photo processor, it contains the latent
images of the exposures that you made. These latent images
must be amplified and stabilized in order to make a color
negative that can then be printed and viewed by reflected
Before we cover the development of a color negative film,
it might be best to step back and process a black-and-white
negative. If you used black-and-white film in your camera, the
same latent-image formation process would have occurred,
except the silver-halide grains would have been sensitized to
all wavelengths of visible light rather than to just red,
green or blue light. In black-and-white film, the
silver-halide grains are coated in just one or two layers, so
the development process is easier to understand. Here is what
- In the first step of processing, the film is placed in
developing agent that is actually a reducing agent. Given
the chance, the reducing agent will convert all the silver
ions into silver metal. Those grains that have latent-image
sites will develop more rapidly. With the proper control of
temperature, time and agitation, grains with latent images
will become pure silver. The unexposed grains will remain as
- The next step is to complete the developing process by
rinsing the film with water, or by using a "stop" bath that
arrests the development process.
- The unexposed silver-halide crystals are removed in what
is called the fixing bath. The fixer dissolves only
silver-halide crystals, leaving the silver metal behind.
- In the final step, the film is washed with water to
remove all the processing chemicals. The film strip is
dried, and the individual exposures are cut into negatives.
When you are finished, you have a negative image of
the original scene. It is a negative in the sense that it is
darkest (has the highest density of opaque silver atoms) in
the area that received the most light exposure. In places that
received no light, the negative has no silver atoms and is
clear. In order to make it a positive image that looks normal
to the human eye, it must be printed onto another
light-sensitive material (usually photographic paper).
In this development process, the magic binder
gelatin played an important part. It swelled to allow
the processing chemicals to get to the silver-halide grains,
but kept the grains in place. This swelling process is
vital for the movement of chemicals and reaction products
through the layers of a photographic film. So far, no one has
found a suitable substitute for gelatin in photographic
If your film were a color negative type (that gives you a
print when returned from the photo processor), the processing
chemistry is different in several major ways:
- The development step uses reducing chemicals, and the
exposed silver-halide grains develop to pure silver.
Oxidized developer is produced in this reaction, and the
oxidized developer reacts with chemicals called
couplers in each of the image-forming layers. This
reaction causes the couplers to form a color, and this color
varies depending on how the silver-halide grains were
spectrally sensitized. A different color-forming coupler is
used in the red-, green- and blue-sensitive layers. The
latent image in the different layers forms a different
colored dye when the film is developed.
- Red-sensitive layers form a cyan-colored dye.
- Green-sensitive layers form a magenta-colored dye.
- Blue-sensitive layers form a yellow-colored dye.
- The development process is stopped either by washing or
with a stop bath.
- The unexposed silver-halide grains are removed using a
- The silver that was developed in the first step is
removed by bleaching chemicals.
- The negative image is then washed to remove as much of
the chemicals and reaction products as possible. The film
strips are then dried.
The resultant color negatives look very bizarre. First,
unlike your black-and-white negative, it contains no silver.
In addition to being a color opposite (negative), the
negatives have a strange orange-yellow hue. They are a
color negative in the sense that the more red exposure, the
more cyan dye is formed. Cyan is a mix of blue and green (or
white minus red). The overall orange hue is the result of
masking dyes that help to correct imperfections in the
overall color reproduction process. The green-sensitive image
layers contain magenta dye, and the blue-sensitive image
layers contain yellow dye.
The colors formed in the color negative film are based on
the subtractive color formation system. The subtractive
system uses one color (cyan, magenta or yellow) to control
each primary color. The additive color system uses a
combination of red, green, and blue to produce a color. Your
is an additive system. It uses small dots of red, green, and
blue phosphor to reproduce a color. In a photograph, the
colors are layered on top of each other, so a subtractive
color reproduction system is required.
Additive and Subtractive
This figure shows a magnified cross-section
of a color negative film exposed to yellow light and
then processed. In the additive system, yellow is red
plus green. On the film, therefore, the red-sensitive
and green-sensitive layers have formed cyan and magenta
Making the Prints
Color negatives are not
very satisfying to look at. They are small, and the colors are
strange to say the least. In order to make a color print, the
negatives must be used to expose the color print paper.
Color print paper is a high-quality paper that is specially
made for this application. It is made waterproof by extruding
plastic layers on both sides. The face side is then coated
with light-sensitive silver-halide grains that are spectrally
sensitized to red, green and blue light. Since the exposure
conditions for a color print paper are carefully controlled,
the paper's layer structure is much simpler than that of the
color negative film. Once again, gelatin plays a key
part as the primary binder that holds the image-forming grains
and the color-forming components (couplers) together in very
thin, individual layers on the paper surface.
Let's start with a black-and-white negative and make a
print. You have the choice of an enlargement or a
direct-contact print. If you want a larger size print than the
original negative, you will need an enlarger, which is
basically a projector with a lens for focusing the image and a
controlled light source. The negative is placed in the
enlarger, and it is projected onto a flat surface that holds
the paper. The image is carefully examined to ensure that it
is in focus. If not, adjustment can be made to the lens and
projection length. Once the size of the image and its focus
are satisfactory, all the lights are shut off, and the
black-and-white paper is placed onto the flat surface. The
paper is exposed for a specified amount of time using the
light from the enlarger. A latent image is formed in the
exposed silver grains. This time, the densest areas of the
negative receive the least amount of light, and therefore
become the brightest and most reflective parts of the prints.
The development process is much the same as for the
black-and-white negative film, except the paper is much larger
than the film, and agitation of the processing chemicals
becomes more critical and more difficult. The final image is
actually developed silver, and by carefully washing the prints
to remove all the unwanted materials, these prints can last a
very long time.
Prints from color negatives are usually done by a large
central lab that handles printing and processing for many
local drug stores and supermarkets, or they may be done
in-house using a mini-lab. The mini-lab is set up to do one
roll of film at a time, whereas the product houses splice many
rolls together and handle a high volume of pictures on a
semi-continuous basis. In either case, the steps are the same
as already discussed for generating a color negative image.
The major difference comes in the printing process, where long
rolls of color paper are pre-loaded into a printer. The roll
of negatives is loaded, and the printer operator works in
normal lights to preview each negative and make adjustments to
the color balance. The color balance is adjusted by adding
subtractive color filters to make the print more pleasing,
particularly when it has been exposed incorrectly. There is
only so much correction that can be done, so don't expect
miracles. Once a full roll of paper is exposed, or a single
roll of film has been printed (in the case of a mini-lab), the
paper is processed.
Here are the steps in developing the color print paper
after it is exposed:
Once again, the gelatin
binder swells to allow the processing chemicals access to the
silver-halide grains, and allows fresh water to rinse out the
by-products. The colored image should contain no residual
- The latent-image sites are developed, and oxidized
developer molecules combine with the color-forming couplers
to create a silver image and a dye image. The reaction is
stopped by a washing step.
- The silver image and any remaining unexposed silver
halide is removed in a combined bleach-plus-fix solution
(called the BLIX).
- The print is then carefully washed to remove any
- The print is dried.
As a final example of color printing process, let's take a
look at our negative that was exposed to a pure yellow object.
When the resultant negative is placed in the printer, and
white light is shown through the negative onto the color
paper, here is what happens. The white light exposure is the
equivalent of a color print exposure. Only blue light gets
through the color negative and exposes the color paper. The
exposed color paper then forms yellow dye in the
blue-sensitive layer, and the original color is reproduced.
This figure shows a magnified cross-section
of a color negative film exposed to white light and then
processed. White light passes through the film to form
blue light, which activates the blue-sensitive layer on
the color print paper to create yellow
If you've made it this far, you are to be congratulated!
Photography isn't as easy as it seems, but then again, that is
what makes it so remarkable. The ability to capture and record
individual photons of light and turn them into a lasting
memory requires many steps. If any one of them goes wrong, the
entire result may be lost. On the other hand, when all the
stuff works, the results are truly astounding.
For more information, check out the links on the next page.
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Mr. Woodworth, or Chuck as he prefers to be
called, grew up in a family that loved to take pictures. When
he graduated from the University of Pennsylvania as a chemical
engineer, he was lured back to his home state of New York to
work for the Eastman Kodak Company. He worked there for 29
years in manufacturing process development, sensitized product
development, product engineering, and as a technical
supervisor. He is now semi-retired in the foot-hills of North
Carolina, where he enjoys driving his 1967 Austin Healey and
occasionally racing his 1959 Alfa Romeo as a member of the
Vintage Sports Car Club of America. He still loves photography
in digital or conventional form. His wife is a teddy bear
artist who has sold her one-of-a-kind creations to customers
around the world.