The ASA or ISO number attributed to film describes the size of the emulsion
grain on that film.
The emulsion grain is the silver halide which oxidizes on
the surface of the film in the presence of light.
The larger the grain on the
surface, the faster the film will be exposed.
With smaller grain on the surface,
you excellent picture quality, but it requires a long period of exposure.
photo microscopy, the light conditions are constant, so choosing a film based
on changing light is unnecessary.
When choosing a film for use in photo
microscopy, you must compromise.
For example, with a choice between three
types of film, Pan-X (ISO 32), Plus-X (ISO 100), and Tri-X (ISO 400), each
has its own benefits.
The Pan-X will produce immensely detailed photos, but
it requires quite a long time for exposure.
The Tri-X film will expose very
quickly, but since the emulsion grain is so large, the pictures will look grainy
and lack detail.
To capture the best pictures, the compromise is to take the
medium Plus-X film.
The emulsion grain is medium, so the pictures are
relatively detailed while keeping the exposure time down.
: explosive, irritant, toxic, flammable
: irritation of the skin, eyes, nose, and throat; difficulty in breathing;
problems with the lungs; delayed reaction time; memory difficulties; stomach
discomfort; and possibly changes in the liver and kidneys