Advanced Photography Course

The original number in table 2-1 is also called an

antilog. Notice that a number greater than one is a
positive log. Any number less than 1, but greater than
zero, is a negative log.

Logs are also required between the numbers 1 and

10. Since the log of 1 is 0 and the log of 10 is 1, the

numbers 1 through 9 are decimals. (See table 2-2.)

Notice the relationships between numbers and

their logs as follows:

When numbers are multiplied, their logs are

added. Example: 8 = 2

4. The sum of log 2 and

log 4 equals log 8: 0.30 + 0.60 = 0.90.

When numbers are divided, their logs are

subtracted. Example: 3 = 6

2. Log 3 is the

difference between log 6 and log 2: 0.78 - 0.30 =
0.48.

The previous discussion is provided to give you a

general idea on how logarithms are derived. It is not
necessary for you to memorize logarithms, or refer to
the log tables. All scientific calculators have a "log"

key that converts numbers to logarithmic form. You
should become familiar with the functions of your
calculator before proceeding with the study of

photographic quality assurance. For more information
on using logarithms, refer to the chapter on logarithms
in Mathematics, Volume 1, NAVEDTRA 10069.

One of the main uses of logarithms in

photographic quality assurance is to take the numbers
used to indicate exposure in characteristic curves and
reduce them to a manageable form. For example, the

sensitometer in your imaging facility is set on an
exposure time of 1/100 second and provides an
illuminance of 80,000 lux (or meter-candles). The log
exposure can be calculated easily as follows:

× =

80,000 (lux)

1/100 (set) = 800 lux seconds

The log exposure = the log of 800 or 2.90

When you convert exposure to logarithmic form,

both density and exposure are on the same scale. A
characteristic curve indicates how exposure and
processing differences affect photographic emulsions
by comparing density and the log of exposure.

To describe sensitometry, you must become

acquainted with several new terms and formulas. As
a starting point, you should become familiar with the
terms transmission, opacity, and density, or T, O, and
D.

TRANSMISSION

Most photographic material, even clear film, does

not transmit all of the incident light that is relevant to
it. Transmission is a measure of the light-passing
ability of a film or other medium. The transmission
of a processed film refers to the fraction, or
percentage, of incident light that passes through the
film.

In a formula, transmission is represented by a

capital letter T. The formula for determining
transmission is as follows:

Table 2-2.--Common Logarithms Between 1 and 10

= Amount of transmitted light

Amount of incident light

The result is never more than 1/1, or 1.00. For

example, when 10 meter-candles (mc) of light are
incident (or falling) to a film and 5 mc is passing
through it, the transmission is as follows: T = 5/10 or
T = 0.50, or 50 percent. When 2 mc is transmitted,
the formula reads T = 2/10 or 0.20, or T = 20 percent.

OPACITY

Opacity is the ability of a medium to absorb light.

The

two terms,

transmission

and

opacity, are

directly

opposite in meaning. Opacity is indicated in a

2-4

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Advanced Photography Course

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