Negative, Positive and Reversal
Some people become confused when negative, positive and reversal films are discussed. Following is a simplified explanation, with no understanding of physics or chemistry required, honest.
Negative films are simple to understand. Where the film is exposed to light, it turns dark after developing. So light objects, photographed on negative film come out dark and dark objects come out light. Also negatives are developed to a low contrast, which means there is less difference between light and dark values in the negative than was present in the scene being photographed.
Positive films are so called because they normally have a positive image on them, that is, one that duplicates the original scene. Actually positive film are essentially the same as negative films except that the contrast is higher tocompliment the contrast of the negative. In printing, where the positive film is exposed to light through the negative it turns dark after developing. The resulting negative of a negative is a positive image.
Reversal films are so called because of the processing they receive in the lab, known as chemical reversal. Without going into technical detail, the results are simple enough. Reversal film gives a result, after processing, that essentially duplicates the original exposure in the camera.
A production photographed with negative film must be printed on positive stock for projection. A reversal film can itself be projected, or if prints are required they can be made on reversal print stock.
Where more than a few prints are wanted it is common to employ specialized intermediate films to enable lower cost prints to be made. Usually a color negative is made from color reversal original, for large quantity release prints on color positive.