light-colored object records as a heavy-density (dark)
area, and a dark-colored object records as a low-density
(thin) area; therefore, a negative image is reversed
compared to the original scene. This reversal is
produced by a bright object in the scene reflecting more
light than a darker object. The greater amount of
reflected light from the brighter object affects more
silver halides in the emulsion.
The portions of the negative where the most silver
halides have been affected are referred to as
HIGHLIGHTS. The portions that are least affected are
referred to as SHADOW AREAS. The light reflections
from objects other than the brightest and darkest are
referred to as MIDTONES.
The amount of metallic silver deposit in any portion
of a negative is referred to as density. The amount of
light that a negative transmits in a given period of time
is controlled by the density of the metallic silver
deposits. Therefore, density is used to describe the
light-stopping ability of a negative.
The difference in densities between areas in a
negative is known as contrast. The total contrast
(density range) of a negative is defined as the difference
in density between the least-dense shadow area and the
most-dense highlight area in a negative.
Emulsion Latitude
The inherent ability of a black-and-white film to
record a range of scene brightness differences as
differences in density is termed emulsion latitude
Normal- and low-contrast emulsions can record a wide
range of scene brightness values and are considered to
have a wide emulsion latitude or long scale.
High-contrast emulsions record a short range of scene
brightness values and are considered to have a narrow
emulsion latitude or short scale.
Exposure Latitude
The amount the exposure can be varied (increased
or decreased) from the ideal exposure and still provide
an acceptable negative is termed exposure latitude.
The least amount of exposure that records sufficient
shadow detail generally provides the best quality
negative and is considered the ideal exposure. The use
of a determined exposure index leads to the ideal
exposure for a particular film and process combination.
Generally, the use of the assigned ISO speed provides
acceptable results.
Emulsion Definition
Several factors, including graininess, resolving
power (resolution), and acutance, affect the definition
or capability of an emulsion to produce a clear, sharp
image.
GRAININESS.
­The negative image consists of
nearly an infinite number of density deposits of metallic
silver. To the naked eye, these grains of silver appear as
a smooth, continuous deposit; however, when the image
is magnified significantly, a speckled, granular, or
mottledeffect becomes apparent. This appearance of the
enlarged image is called graininess.
The graininess of a negative depends upon the size
of the silver halides in the emulsion, the exposure the
emulsion received, and the clumping action of the
metallic silver grains during development. Although
each emulsion has an inherent grain size, the graininess
effect in a negative can be minimized by giving the film
correct exposure and proper development. As a general
rule, slower speed emulsions have a finer grain
(a smaller grain size) than faster speed emulsions.
The Eastman Kodak Company uses modem tabular,
or T-grain, technology in some of their emulsions. In
doing so, flat, tabular crystals are used which are very
efficient in absorbing light. In T-grain emulsions, fewer
silver halide crystals are needed, because the crystals
have a relatively large surface area for light to strike, but
are thin and contain a small amount of silver halides.
These properties provide higher resolution and lower
graininess compared to other films with the same
relative film speed.
Not all black-and-white films form the final image
in black metallic silver; for example, Ilford's XP-1
black-and-white film produces a dye image. The
advantage of this technology is the film can be processed
in color-processing chemicals along with color film and
then printed on black-and-white paper.
RESOLVING POWER.
­This is a term used to
define the ability of an emulsion to record fine detail.
The resolving power, or resolution, of an emulsion is
expressed as line pairs per millimeter. Resolving power
is determined by photographing a lined test chart at a
greatly reduced size then counting the lines present in
one millimeter of film.
ACUTANCE.
­This is the ability of an emulsion to
produce sharp edges between image density differences.
Do not confuse this with the ability of an emulsion to
produce sharp images controlled by lens focus.
Although an image is in focus, the line between
a
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