What is film?
Film is, very simply, a light-sensitive emulsion on a plastic base.
An easy way to think of film is to compare it with bread and butter. Think of the bread as the base, the butter as the emulsion. When you hold this combination in your hand, what you feel and see is mostly bread, the base -- not butter, the emulsion. The base (bread) holds and supports the emulsion (butter), the active part of the film.
As butter gives your bread flavor, emulsion gives your film an image-recording ability. Emulsion is a gelatin made of millions of light-sensitive silver halide crystals. When you open your camera shutter and light strikes these crystals, a reaction takes place -- preserving on film the image of what the camera has seen.
But you can't see that image -- yet. In fact, if you could examine the emulsion before and after exposure to light, you would see no difference. Although the image is there, it's latent. It has to be processed before it becomes a picture that you can see.
How does film work?
Let's make it simple and first consider black-and-white film. When you take a picture here's what happens.
Light...reflected off a subject...passes through a lens...onto film
Snap! The shutter opened and closed for only a fraction of a second. But that was enough time for light to burst through and strike the emulsion. And an important change has occurred -- the silver halide crystals that were struck by light have been rearranged. Later, when the film is placed in a developer solution, the light-struck silver halide crystals react chemically with the developer to form black grains of silver which remain in the film.
After the developer is removed and the film is rinsed, a chemical fixer is added. The fixer removes the crystals which were not exposed to light. What is left of your original film is now called a developed negative, just the ones you receive when you get your prints back.
A negative is a reverse image of the picture you took. The blackest areas show where the densest concentration of black silver grains are -- where the greatest amount of light struck the emulsion. The lightest parts of the negative -- where there is little or no black silver at all -- show where the scene had been darkest and had reflected no light onto the emulsion. All other areas are in shades of gray, varying in proportion to the brightness of each individual subject in the screen.
But you can't enjoy a negative. You need a positive image. To get it, we pass light through the negative and onto photographic paper. This is a special paper coated with light-sensitive silver compounds -- just like film. And the exposure of your negative upon photographic paper is called printing.
Why do some films produce color pictures?
Instead of just one layer of emulsion, as in black-and-white, color film has several layers, each emulsion recording a different color. Between the emulsions are protective interlayers and all of these layers together aren't as thick as a human hair. That gives you can idea of the care needed to make color film -- and the delicate handling needed to process it properly.
Negative film -- Reversal film
How do they differ?
Negative color film -- like black-and-white film -- is used to print the finished picture on photographic paper. When you send negative color film -- KODAK ROYAL GOLD Film -- you get back color prints.
With reversal film, your photofinisher makes a full-color transparent image on your original film -- KODACHROME or EKTACHROME Slides and Movies.
Reversal films contain three separate color pictures in three layers on one base. After processing, the top layer has a positive yellow image of everything in your picture that is yellow or needs yellow in its finished color. The middle layer is a magenta (purple-red) image, and the bottom is cyan (blue-green). The three layers, when viewed together, form a complete color transparency with infinitely varied and delicate shades.
As for negative film ...
The top emulsion layer of negative film is sensitive to blue light. Beneath this layer is a filter coating which stops the blue light from penetrating deeper. Lower layers are green-sensitive and red-sensitive.
A color negative will be dark where the subject was light, just like a black-and-white negative. Moreover, a negative's color will be opposite that of the subject. For example, if the subject is red, the negative will be greenish-blue.
Some of these relationships are difficult to see behind the characteristic dull-orange tint that cover KODAK ROYAL GOLD Film negatives. But don't worry, these color relationships get straightened out when the image from the color negative is printed on the color photographic paper.