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.
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 =
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.
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
Thus the range of exposure lies within a narrow limit
that may be less than one-half to one f/stop.
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
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
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.
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,
responds to changes in exposure and processing.
Some more useful definitions include the following:
contrast reproduced in an image.
of development (for a given material).
straight-line section of the characteristic curve.
of the most common methods are as follows:
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:
Advanced Photography Course