Advanced Photography Course

The same factors that affect high density affect

low density.

PLOTTING SPEED POINT ON

A PROCESS CONTROL CHART

The speed point (SP) is a measure of the effective

film speed or exposure index of a film. The speed
point is determined by sensitometric tests. The speed
point is established using a step on a sensi-strip with
a density of 0.10 above gross fog for ground pictorial
film. The speed point of aerial film is established by
using the step on a sensi-strip that has a density of
0.30 above gross fog.

Once the speed-point step is determined, that step

is read in successive sensi-strips and plotted on the
control chart. Neither effective film speed nor the
ISO for ground pictorial film should be confused with

effective aerial film speed because they are not
equivalent.

PLOTTING GROSS FOG ON
A PROCESS CONTROL CHART

Gross fog (B+F) is read from a "clear" area of a

control strip; that is, an area that does not receive
exposure. All films have a gross fog density,
resulting from several factors that may include the
following:

The density of the film base

Chemical fog

Age fog

The development of unexposed silver halides

Inadequate fixation (film not cleared)

As stated earlier, the amount of information you

use to monitor or control your process depends on
several factors. However, when you choose to
monitor more than one processing variable, you
should construct the appropriate control chart or use
a piece of graph paper that can be posted near the
process. Figure 2-14 shows a typical family of
control charts for a process. A family of control
charts, such as this, will provide you with a wealth of
information about the process. Also, all the
information is in one place.

LIMIT LINES

The upper- and lower-limit lines on a control chart

are based on the assumption that the plotted points are
representative of a normal "population" or set of
circumstances of the process. The limit lines,
therefore, should include between them, all points
representing an unchanged or normal process. Limit
lines can never be placed in such a manner that all
data are included between them; there will always be
deviations. Samples from a black-and-white process,
for example, show a gamma average of 0.70. On a
subsequent test, a sensitometric strip was found to
have a gamma of 0.80. Obviously this process
appears to have changed or is changing. Should the
process be altered? The answer must consider the
factor of probability.

Two risks are involved in judging whether normal

limits are exceeded. One risk occurs when a certain
sampling appears outside one of the limit lines,
indicating that the process is out of control, but the
process is actually behaving normally and has not
changed. This situation is known as the alpha risk.
The reverse is also possible; it appears that the process
is normal when actually it has changed or is changing.
This is called a beta risk. These occurrences cannot
be eliminated, but they can be reduced to the point

where the probability of their happening is small.
One risk is usually more costly than the other, and the

limits are set accordingly. The limits are set far from
the mean when the alpha risk must be avoided. They
are set close to the mean when the beta risk must be

avoided.

It is standard practice in black-and-white

processing to place the limit lines at three times the
standard deviation above and below the mean, or ą3s.
The alpha risk is approximately 3 in 1,000 for limits
of ą3s. Before proceeding, it is necessary to define
the following two terms:

Population--all possible results (happenings)
in a certain process

Variability--the amount of departure of

measurements (parts of the population) from
the mean (average)

Variability may be expressed in the following

ways:

2-26

contents

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 -> The same factors that affect high density affect low density. PLOTTING SPEED POINT ON A PROCESS CONTROL CHART The speed point (SP) is a measure of the effective film speed or exposure index of a film. The speed point is determined by sensitometric tests. The speed point is established using a step on a sensi-strip with a density of 0.10 above gross fog for ground pictorial film. The speed point of aerial film is established by using the step on a sensi-strip that has a density of 0.30 above gross fog. Once the speed-point step is determined, that step is read in successive sensi-strips and plotted on the control chart. Neither effective film speed nor the ISO for ground pictorial film should be confused with effective aerial film speed because they are not equivalent. PLOTTING GROSS FOG ON A PROCESS CONTROL CHART Gross fog (B+F) is read from a "clear" area of a control strip; that is, an area that does not receive exposure. All films have a gross fog density, resulting from several factors that may include the following: The density of the film base Chemical fog Age fog The development of unexposed silver halides Inadequate fixation (film not cleared) As stated earlier, the amount of information you use to monitor or control your process depends on several factors. However, when you choose to monitor more than one processing variable, you should construct the appropriate control chart or use a piece of graph paper that can be posted near the process. Figure 2-14 shows a typical family of control charts for a process. A family of control charts, such as this, will provide you with a wealth of information about the process. Also, all the information is in one place. LIMIT LINES The upper- and lower-limit lines on a control chart are based on the assumption that the plotted points are representative of a normal "population" or set of circumstances of the process. The limit lines, therefore, should include between them, all points representing an unchanged or normal process. Limit lines can never be placed in such a manner that all data are included between them; there will always be deviations. Samples from a black-and-white process, for example, show a gamma average of 0.70. On a subsequent test, a sensitometric strip was found to have a gamma of 0.80. Obviously this process appears to have changed or is changing. Should the process be altered? The answer must consider the factor of probability. Two risks are involved in judging whether normal limits are exceeded. One risk occurs when a certain sampling appears outside one of the limit lines, indicating that the process is out of control, but the process is actually behaving normally and has not changed. This situation is known as the alpha risk. The reverse is also possible; it appears that the process is normal when actually it has changed or is changing. This is called a beta risk. These occurrences cannot be eliminated, but they can be reduced to the point where the probability of their happening is small. One risk is usually more costly than the other, and the limits are set accordingly. The limits are set far from the mean when the alpha risk must be avoided. They are set close to the mean when the beta risk must be avoided. It is standard practice in black-and-white processing to place the limit lines at three times the standard deviation above and below the mean, or ą3s. The alpha risk is approximately 3 in 1,000 for limits of ą3s. Before proceeding, it is necessary to define the following two terms: Population--all possible results (happenings) in a certain process Variability--the amount of departure of measurements (parts of the population) from the mean (average) Variability may be expressed in the following ways: 2-26 contents Advanced Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
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