Another similarity between light and rain on a
sidewalk is in the blackening effect. With light the
blackening (during development) increases with the
exposure received by the sensitive film emulsion. The
photographic lens and shutter assembly should be
regarded as a device that controls the camera exposure
received by the light-sensitive film emulsion inside the
camera.
The aperture of the lens diaphragm controls the
intensity of the light, and the shutter controls the time of
exposure. Since a photographic reproduction of the
original scene contains a range of tones of different
brightness, a corresponding range of photographic
exposure is given to the sensitized emulsion.
FILM LATITUDE
A negative is said to be correctly exposed when it
gives a satisfactory rendition of detail in both the deepest
shadows and brightest highlights of the scene or subject.
Fortunately, in many cases, there is more than a single
exposure that will produce this result — there is a wide
range of possible exposures within which satisfactory
tone separation is possible. The “minimum” satisfactory
exposure is one in which good tone separation is just
attained in the deepest shadow areas. The “maximum”
satisfactory exposure is one in which detail is just
retained in the brightest highlight. Any additional
exposure will cause this highlight detail to become
flattened out or “blocked up.”
The range between these two exposures is known
as latitude. This latitude may be narrow or wide,
depending on the subject matter, lighting contrast, type
of film and degree of development of the negative. In
general, the black-and-white films you will be using
have a greater margin for error than color films. Ignoring
the influence of development for a moment, a softly
lighted scene composed of objects that are, themselves,
fairly uniform intone will allow a wide range of possible
exposures that will produce a satisfactory rendition on
the negative. This is caused by the narrow range of tonal
values from highlight to shadows in the subject. On the
other hand, a brilliantly lighted scene composed of a
variety of tones, from jet black to snow white, may take
up the entire usable range of the negative scale.
Therefore, the exposure required for the proper
rendition of the entire range of tonal values in this scene
may be quite critical.
In addition there are many scenes, such as interiors
with sunlight coming through a window, that have a
range of brightness so wide that no single exposure can
produce both highlight and shadow detail in a
black-and-white negative. When you increase exposure
and reduce the amount of development, almost any
ordinary extreme of brightness range can be
accommodated on black-and-white film.
FILM SPEED
The sensitivity of black-and-white and color film
for still-camera use is also called the film speed, the ISO
speed or simply the ISO. Earlier in this chapter, we
pointed out that “ISO” is an acronym for International
Standards Organization, a federation of all national
standard bodies of the world, which has approved a
uniform set of film-speed standards. These standards
call for a universal expression of both arithmetic and
logarithmic values with the ISO designation.
Until early 1983 the emulsion speeds of still-camera
film were expressed in ASA values (which are
arithmetic) or in DIN values (which are logarithmic).
ASA values were determined according to standards
published by the American National Standards Institute,
formerly American Standards Association from which
the designation ASA came. The DIN values reflected the
German standards established by the Deutsche Industrie
Norm.
Film speed is determined by the manufacturer
according to the ISO standards. It will generally look
like this:
ISO 100/21°
The number immediately following “ISO” is the
ASA equivalent. It indicates that the speeds progress
arithmetically, and any film marked ISO 100 has the
same sensitivity as any other film marked ISO 100 — it
is twice as fast as film marked ISO 50 and is half as fast
as film marked ISO 200. The number with the degree
symbol (0) is the DIN equivalent.
The arithmetic speed number is intended for
exposure meters or cameras marked for ISO or ASA
speeds or exposure indexes. The logarithmic speed is
intended for exposure meters or cameras marked for
ISO or DIN settings.
FILM TYPES
Photographic films (and papers) are composed of
two basic parts: the emulsion and the base, or support.
The emulsion is the light-sensitive portion of a film or
paper that records the image. The emulsion contains the
silver halides and any special sensitizing dyes
suspended in a binder of gelatin. The gelatin holds the
silver halides evenly dispersed and prevents action by a
developer until the silver halides have been made
developable either by exposure to light or chemical
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