Advanced Photography Course

CONTRAST INDEX

As you know, "contrast" means the range of

densities produced by a combination of subject
luminance ratio and the amount of development. The
luminance ratio of the scene cannot be controlled
outdoors and can be controlled only within limits in
the studio. When the degree of development is
controlled, one variable that affects negative contrast
is practically eliminated. When films are developed
to a given contrast index (CI) value, they can be made
compatible with any printing system. Variations in
contrast caused by different subject luminance ratios
are then adjusted in printing by selecting the
appropriate variable contrast printing filter. Controlled
film development helps to produce a uniform standard
of print quality, as well as to make printing easier and
less costly in both time and materials.

Contrast index has the same uses as gamma. It is

an index number that can be used for processing
control purposes. When films or developers are
compared, the test images should all be developed to
the same contrast index. Obtaining a constant contrast
index, like gamma, does not guarantee that all
negatives will have the same total contrast or that they
will all print similarly.

The contrast index is measured over that part of

the D-log H curve used in exposing continuous-tone
negatives correctly. CI is the average of the slope
and is distinctly different from the straight-line slope
of gamma. In this context, an average is the slope of
a straight line drawn between two definite points on
the curve. The straight line is drawn between two
points on the D-log H curve that represents the
highest (D-max) and the lowest (D-min) useful
densities on the characteristic curve.

To determine the contrast index, you must use a

transparent overlay on the D-log H curve. Refer to
figure 2-9 at this time. To use this transparent
contrast-index gauge, you must place it on the gross
fog density line of the characteristic curve. The gauge
is then moved right or left until the low-density arc
intersects the toe of the curve, and the high-density
arc on the shoulder of the curve reads the same value.
This value is the contrast index. Figure 2-9 shows the

proper use of a contrast-index gauge.

Since contrast-index gauges are not found in most

imaging facilities, an alternative method is used to
determine the contrast index of a characteristic curve.
This method is not as accurate as using a
contrast-index gauge, but provides an approximate
value. To use this method, first locate the density
point (in the toe area of the characteristic curve) that
is 0.10 above B+F. Then, using a compass, align it
on the log-H axis and spread it to a distance equal to
2.0 in logs. Place the point of the compass on the
density point that is 0.10 above B+F and draw an arc
on the curve. Finally, draw a straight line between

the two points and determine the slope, using the
same formula you used for gamma. The result is the
approximate contrast index.

CONTROL IN PROCESSING

The gamma or contrast-index value of a

photographic material is not fixed. Both vary,
depending on the process used. It is important for
you to realize that the contrast obtained by
development depends on the amount of development,
rather than on the development time alone. To obtain
an accurate gamma or contrast index, you must
control the total amount of development carefully,
such as developer temperature, developer strength,
degree of agitation, and other variables.

The densities of a characteristic curve are changed

by changes in development. Therefore, gamma and
contrast-index values are useful as a measure of the
degree of development. Gamma and contrast index
vary directly with the degree of development; the
greater the development, the higher the gamma or
contrast index. This is true until the film is grossly
overdeveloped. When film is grossly overprocessed,
the contrast begins to decrease, because the unexposed
silver halides have developed (increased gross fog)
after the maximum density has "peaked" out. The
point that the gamma or contrast index reaches its
maximum level is known as gamma (or CI) infinity.

Gamma and contrast index are two important tools

used in processing control. Films developed to the
same value, for example, show comparable tone
reproduction. When you want to determine whether
processing is consistent, sensitometric strips are

2-17

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Advanced Photography Course

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DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> CONTRAST INDEX As you know, "contrast" means the range of densities produced by a combination of subject luminance ratio and the amount of development. The luminance ratio of the scene cannot be controlled outdoors and can be controlled only within limits in the studio. When the degree of development is controlled, one variable that affects negative contrast is practically eliminated. When films are developed to a given contrast index (CI) value, they can be made compatible with any printing system. Variations in contrast caused by different subject luminance ratios are then adjusted in printing by selecting the appropriate variable contrast printing filter. Controlled film development helps to produce a uniform standard of print quality, as well as to make printing easier and less costly in both time and materials. Contrast index has the same uses as gamma. It is an index number that can be used for processing control purposes. When films or developers are compared, the test images should all be developed to the same contrast index. Obtaining a constant contrast index, like gamma, does not guarantee that all negatives will have the same total contrast or that they will all print similarly. The contrast index is measured over that part of the D-log H curve used in exposing continuous-tone negatives correctly. CI is the average of the slope and is distinctly different from the straight-line slope of gamma. In this context, an average is the slope of a straight line drawn between two definite points on the curve. The straight line is drawn between two points on the D-log H curve that represents the highest (D-max) and the lowest (D-min) useful densities on the characteristic curve. To determine the contrast index, you must use a transparent overlay on the D-log H curve. Refer to figure 2-9 at this time. To use this transparent contrast-index gauge, you must place it on the gross fog density line of the characteristic curve. The gauge is then moved right or left until the low-density arc intersects the toe of the curve, and the high-density arc on the shoulder of the curve reads the same value. This value is the contrast index. Figure 2-9 shows the proper use of a contrast-index gauge. Since contrast-index gauges are not found in most imaging facilities, an alternative method is used to determine the contrast index of a characteristic curve. This method is not as accurate as using a contrast-index gauge, but provides an approximate value. To use this method, first locate the density point (in the toe area of the characteristic curve) that is 0.10 above B+F. Then, using a compass, align it on the log-H axis and spread it to a distance equal to 2.0 in logs. Place the point of the compass on the density point that is 0.10 above B+F and draw an arc on the curve. Finally, draw a straight line between the two points and determine the slope, using the same formula you used for gamma. The result is the approximate contrast index. CONTROL IN PROCESSING The gamma or contrast-index value of a photographic material is not fixed. Both vary, depending on the process used. It is important for you to realize that the contrast obtained by development depends on the amount of development, rather than on the development time alone. To obtain an accurate gamma or contrast index, you must control the total amount of development carefully, such as developer temperature, developer strength, degree of agitation, and other variables. The densities of a characteristic curve are changed by changes in development. Therefore, gamma and contrast-index values are useful as a measure of the degree of development. Gamma and contrast index vary directly with the degree of development; the greater the development, the higher the gamma or contrast index. This is true until the film is grossly overdeveloped. When film is grossly overprocessed, the contrast begins to decrease, because the unexposed silver halides have developed (increased gross fog) after the maximum density has "peaked" out. The point that the gamma or contrast index reaches its maximum level is known as gamma (or CI) infinity. Gamma and contrast index are two important tools used in processing control. Films developed to the same value, for example, show comparable tone reproduction. When you want to determine whether processing is consistent, sensitometric strips are 2-17 contents Advanced Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
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