or print. The color sensitivity determines how the film
is classed. There are four general classes of
black-and-white film emulsions. The four classes are as
follows: colorblind (monochromatic), orthochromatic,
panchromatic, and infrared. Some of these emulsions
respond to a wide range of wavelengths of light. Others
respond to only a narrow band of wavelengths.
Light-sensitive emulsions are sensitive to all
wavelengths of ultraviolet radiation. For all practical
purposes, the general classes of emulsions are
considered insensitive to the shorter wavelengths of
ultraviolet (UV) radiation. This is because glass lenses
and the gelatin in most film emulsions completely
absorb the shorter wavelengths of ultraviolet radiation.
When UV is to be used for photography, a special film
with a thin emulsion is required.
Color-blind Emulsions.
­Black-and-white color-
blind emulsions are sensitive only to UV radiation,
violet, and blue light. Green and red objects record only
as clear areas in the black-and-white negative and
reproduce as dark areas in the print. Color-blind films
are used primarily for copying and graphic arts
photography and may be assigned three or more ISO
values; for example, ISO/50 for daylight, ISO/8 for
tungsten light, ISO/20 for white-flame arcs, and ISO/12
for pulsed xenon.
Orthochromatic Emulsions.
­Orthochromatic
emulsions are sensitive to ultraviolet radiation, violet,
blue, and green light. The sensitivity to green light is
gained by the addition of a sensitizing dye to the
color-blind silver halides. The emulsions provide an
approximate correct reproduction of blue and green
objects as corresponding tones of gray in a print;
however, red objects record as clear areas in the negative
and black areas in the print. since the emulsion is not
sensitive to red. Various orthochromatic films with
different degrees of contrast, color sensitivity, and
emulsion speed are available. Their trade names usually
contain the word ortho. Orthochromatic emulsions are
used primarily for copying and graphic arts
photography.
Orthochromatic emulsions that may be used in
either daylight or tungsten light are assigned two
separate ISO film speeds. This is because these
emulsions are highly sensitive to the predominantly blue
colored daylight and less sensitive to the tungsten light
that has a higher content of red light.
Panchromatic Emulsions.
­Panchromatic emulsions
are sensitive to UV radiation, violet, blue, green, and
red. The emulsion spectral sensitivity to green and red
light is gained by adding sensitizing dyes to the
color-blind silver halides. Panchromatic film of varying
degrees of contrast, color sensitivity, and emulsion
speed is available. Panchromatic emulsions are used for
copying, portraiture, and general black-and-white
photography.
Panchromatic emulsions are assigned only one IS0
film speed. This is because panchromatic emulsions are
sensitive to red light and have an almost equal response
to predominately blue-colored daylight and
predominately red-colored tungsten light.
Infrared Emulsions.
­Infrared (IR) emulsions are
sensitive to UV radiation, violet, and blue light, with
little or no sensitivity to yellow-green light but with
additional sensitivity to red and IR radiation. The
sensitivity to infrared radiation is gained by adding a
sensitizing dye to the color-blind silver halides. Infrared
emulsions are commonly used for aerial and medical
photography as well as forensic photography
(photography used for evidence). For best results a
black-and-white UV film should be exposed only with
IR radiation. To prevent any IR radiation or visible light
from affecting the infrared emulsion during exposure,
you must use a dark, red filter over the camera lens.
Since infrared radiation does not focus at the same
point as visible light, a lens focus adjustment is
necessary for critical focusing. Most lenses have a
calibrated infrared focusing position on the focusing
scale. This position is usually marked by a small, red dot
or the letter R in red.
Determining a useful exposure index becomes a
problem with infrared film, because exposure meters are
calibrated for visible light and similar light sources can
emit different amounts of infrared radiation. When using
infrared film, you should make trial exposures for each
particular film and photographic situation.
Contrast
In the development process, the silver halide grains
in a black-and-white film exposed to light remain in the
film. These grains form the image of the original scene.
The colors of the scene are recorded in the negative as
densities of gray instead of appearing as their original
colors. These densities of gray can range from very
dense to very thin. This depends upon the brightness of
the objects in the scene, their color, and the color
sensitivity of the film. The ratio of the maximum to the
minimum brightness of objects in a scene is referred to
as the scene brightness range. Most long scale
black-and-white films are capable of recording scene
brightness ranges up to 128:1. In a negative, a
2-3

Basic Photography Course












Privacy Policy