much lower upper limit because of the high densities
involved
Exposure Latitude
Exposure latitude is the allowable range of
exposures for a given photographic emulsion. It
varies with the brightness range of a scene. Film with
a wide exposure latitude permits a greater variance of
exposure and still produces an acceptable negative.
Exposure latitude is associated with the useful
log-exposure range and can be thought of as the
margin of camera exposure error. Scenes that have a
relatively low-luminance ratio and are photographed
using a long-scale film have more exposure latitude
than a scene with a high-luminance ratio using the
same film. For example, you are photographing a
subject with a luminance ratio of 60:1. This subject
requires a log-exposure range of 1.78 (log of 60 =
1.78). The useful log-exposure range of the
film/development combination is 2.70. This leaves a
difference in logs of 0.92 (2.70 - 1.78 = 0.92). In
this example, the exposure latitude is about three
f/stops (remember, one f/stop = 0.30). Normally, the
lower the contrast of the scene and the faster the film,
the greater is the exposure latitude.
Modem high-speed films have an overall exposure
latitude of several stops for an average subject.
However, regardless of the brightness range of the
scene, color reversal and very slow black-and-white
and color negative films have very little exposure
latitude because of their increased inherent contrast.
Thus the range of exposure lies within a narrow limit
that may be less than one-half to one f/stop.
Gamma
In technical terms, gamma (signified by the Greek
letter
g)
is a sensitometric quantity that indicates the
slope or gradient of the straight-line section of the
characteristic curve. It is interpreted as a measure of
the contrast reproduced in a negative image; that is,
the ratio of negative contrast to original subject
contrast for a given range of tonal values. It measures
the degree of development of photographic materials,
since changes in development affect contrast or affect
the slope of the curve. Exposure changes, as
explained previously, shift the position of the points
right or left on the log-H axis without altering the
slope of the curve. Thus the tendency is for exposure
to control the density and development to control the
contrast of the image reproduced. Remember the
expression, "Expose for shadow density—develop for
contrast."
Mathematically, gamma is the ratio of height
gained to distance traveled in a horizontal direction.
In determining gamma, the height is density (D), and
the horizontal base is log exposure (log H). An ideal
film and processing might produce an increase of .3
density for each .3 increase of exposure. This ratio is
0.3:0.3, or 1.0.
Most ground pictorial subjects call for film with
a gamma value around 0.75, varying from 0.65 to
0.90. Such emulsions record the wide range of tones
present in outdoor scenes. In practice, each of the
main groups of negative materials has its own
individual characteristics. Gamma is useful to you,
because it indicates how the photographic material
responds to changes in exposure and processing.
From the previous discussion, you may have
noticed that gamma is definable in different ways.
Some more useful definitions include the following:
Gamma is the numerical measure of the
contrast reproduced in an image.
Gamma is a numerical measure of the degree
of development (for a given material).
Technically, gamma is the slope of the
straight-line section of the characteristic curve.
Once a characteristic curve has been plotted,
gamma can be determined in a number of ways. Two
of the most common methods are as follows:
Basic method—This method shown in figure
2-7 involves the ratio between densities and the
exposures that produced them. Any two points on the
straight line are chosen. (More reliability results when
the points are widely separated.) Gamma is the result
of dividing the change, or difference in density, by the
difference in log H between the two points. The
formula is as follows:
g =
where
D
D
D
log H o r
D1 - D2
log H1 - log H2
D
(Delta) = Symbol for change or difference
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