Camera film is made of very fine silver halide crystals suspended in gelatin that is coated on a light absorbent backing. The basic premise that allows the use of photographic emulsions to form lasting images is the light sensitive characteristics of these silver halide crystals. Film is developed through a process of eliminating the photosensitivity of the film emulsion and chemically removing the unexposed silver crystals.
The simplest type of film is black-and-white film. It forms images based upon the brightness of the image formed by the camera's lens. Contrast is produced by variations in the relative brightness of areas in the image to each other.
Color film is more complicated. It is made possible because of the color separating qualities inherent in filters. When light is shone through a colored filter, light of the same color as the filter is transmitted more readily than others. Color film is made of three layers, with each layer being dyed with different primary color. A red image is produced on the red layer from red light in the subject, a green image on the green layer, etc. When slides are projected, or prints made, the three colors combine in the same amounts as in the original image forming the illusion of a full color spectrum to our eyes.
Among each type of film there are different "speeds." Film is normally manufactured with speed variations measured in stops or fractions of stops with each full stop being greater or smaller by a factor of two as in 100 speed, 200 speed, etc. Higher speed films are generally less colorful and more grainy than slow films, the difference being especially noticable with film speeds above ISO 400. Slower films tend to show greater resolving capability, more saturated colors, and less visible graininess.
A film will also have a certain exposure latitude. This refers to the range between the brightest (detail lost in white) portions of an image compared to the darkest (detail lost in black) portions of an image in relation to the actual range of stops in the lighting present in those areas when the photograph was taken. Typically, print film will have an exposure latitude of roughly 7 stops and slide film has a range of approximately 5.
What this Means for a Photographer
Exposure latitude matters because your pictures should have contrast within your film's capability. For example, you wouldn't be able to take a picture of the Statue of Liberty and the sun at noon and have your film's latitude "soak up" the extreme range of contrast. Also note that fast film has a greater allowance for exposure error and greater latitude than slow film. Slow speed films are more sensitive to careful exposure adjustments and allow more control over the precise exposure "mood" of a photograph. Also note that because of slow film's higher resolution—which translates into better picture quality—as well as its better color, the slowest film conditions allow is always preferred for the maximum in quality.
When calculating the correct exposure, you must also take into account possible exposure variations that could occur because of a phenomenon known as reciprocity failure. Reciprocity failure has to do with film losing some of its speed and/or devloping a color cast at very fast or very slow exposures. (Exposures within 1/10,000 to 10 sec. are safe* with most film.) Film manufacturers usually can supply charts that show the safe shutter speed ranges, exposure correction factors, and any corrective filtration needed to produce accurate exposure and color for their lines of film under less-than-ideal conditions.
*Tungsten slide film is an exception. Exposures faster than 1/60 sec. show color casting.
© 1999 Joseph K. Myers
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