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An understanding of exposure in relation to wet photography and ISO setting will help you to understand the significance of ISO settings to digital photography.

Film Speed

The "speed" of a film is determined by how fast the film reacts to light.  The more responsive an emulsion is to light, the faster it is said to be.  This means that less exposure to light is required to produce an image. 

The silver halide grains determine how fast the film reacts to light.  Some of these grains are highly sensitive to light while others are less sensitive.  Varying the proportion of highly sensitive grains will affect the speed of the film.

Several systems have been developed to accurately compare the light sensitivity of different films.  One of the most commonly used systems was developed by the American National Standards Institute (ANSI), previously known as the American Standards Association (ASA).  This same system is used by the International Standards Organization (ISO).  This system uses a speed rating in which numbers are assigned to emulsions to indicate their relative speeds.  Doubling of speed is denoted by doubling of the numbers.  For example, a film with a speed of 250 is twice as fast as one with 125; therefore, it takes half the amount of exposure to produce an image.  Digital cameras use electronic ISO calibration settings.

These rating numbers can be seen on boxes of film with the letters ASA/ISO beside them.  T-Max film, for example, would have ASA/ISO 400 on the box and Plus X Pan has ASA/ISO 125.  Relatively speaking, films are termed either slow, medium or fast.  An ASA/ISO of 40 would be a slow film, and ASA/ISO 125 would be medium.  An ASA/ISO of 400 would be fast.  Some films are extremely fast -- with speeds of ASA/ISO 3,200.  The speed of the film you select or ISO setting of a digital camera is determined by the photographic assignment. 


During manufacturing, the halide grains clump together.   This determines the size of the grain in any film, and is called "inherent grain size."  Films can be broadly classified into low, medium, and coarse-grained emulsions.  High-speed emulsions generally have larger grains than slow-speed emulsions and in digital photography, higher ISOs produce grainier images. Large grain size is undesirable except to achieve a special effect. However, they usually will not become apparent until you begin to make enlargements from your negative.   You should use the slowest film, or lowest ISO, possible under the exposure conditions for optimum results.


Contrast is the term used to describe how film will record the difference in tones between the lightest and darkest areas in the negative.  This capability, which is also determined by the size of the grain, is incorporated into the film at the time of manufacture and is described as low, normal and high contrast.   Normal contrast materials have the ability to form a wide range of grays and are used for a majority of general-purpose photography.

The shades of gray as they are reproduced in the negative and its subsequent print can be changed by factors other than the inherent contrast of the film.  Such things as lighting, exposure, development and the developer can produce a negative that may be flat (lacking contrast) or hard (too contrasty).  You can see an example of hard and flat contrast by adjusting the contrast dial of a T.V.


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