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IS0
I N T ERN AT1 O N A L ORGAN IZAT I O N FOR STA N DA RD IZAT I O N
IS0 RECOMMENDATION
R6
METHOD FOR DETERMINING
PHOTOGRAPHIC SPEED
AND EXPOSURE INDEX
ISf EDITION
October 1955
COPYRIGHT RESERVED
The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
This IS0 Recommendation R 6 was prepared by Technical Committee
ISO/TC 42 -Photography, the Secretariat of which is held by the American
Standards Association, Inc. (ASA).
In April 1948, the Secretariat asked the General Secretariat to submit, as
a Draft Proposal, for study by the Technical Committee, the American Standard
Z 38.2.1. - 1947, concerning the method for determining photographic speed
and exposure index. This Draft Proposal was distributed on 12 May 1948 to
all the Members of the Technical Committee. In view of the fact that no
observations were received, the Secretariat considered the Draft Proposal as
adopted by the Technical Committee as a Draft IS0 Recommendation.
In January 1951, this Draft IS0 Recommendation was submitted to all the
v
IS0 Member Bodies by the General Secretariat. The following 15 Member
Bodies (out of the existing total of 29) sent in their approval:
Australia Italy Sweden
Austria
Mexico Union of South Africa
Belgium Netherlands United Kingdom
Czechoslovakia New Zealand U.S.A.
Finland Portugal Yugoslavia
One Member Body stated that it had no objection to the approval of the
Draft:
Denmark
The ISOiTC42 Secretariat amended the Draft, taking account of the
observations made by various Member Bodies.
The revised text was then submitted by correspondence to the IS0 Council
Members who decided, in May 1954, to accept it as an IS0 RECOMMENDA-
TION. J
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lSO/R 6 . 1954 (E)
IS0 Recommendation R6 May 1954
METHOD FOR DETERMINING
PHOTOGRAPHIC SPEED AND EXPOSURE INDEX
CONTENTS
Page
Introduction . 4
for Determining Photographic Speed and Exposure
IS0 Recommended Method
Index . 5
1 . Scope . 5
2 . Discussion of Problem and Explanation of Terms . 5
2.1 Concept of Speed for the Materials in this Specification . 5
2.2 Sensitometric Criterion of Speed . 5
2.3 Exposure Index . 6
3 . Determination of IS0 Recommended Speed and IS0 Recommended Exposure
Index of a Specific Sample . 6
3.1 Determination of Speed . 6
3.2 Determination of Exposure Index . 6
3.3 Testing Technique . 7
4 . Determination of IS0 Recommended Speed and IS0 Recommended Exposure
Index of a Product . 10
4.1 Speed and Exposure Index of a Product . 10
4.2 Sampling . 10
4.3 Storage of Samples . 10
4.4 Testing . 10
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ISO/R 6 - 1954 (E)
INTRODUCTION
This recommendation describes a sensitome- is based was first suggested by experimental
tric method for determining and expressing the work reported in various technical journals.*
photographic speed of roll films, film packs, A quantity called “Exposure Index” is specified
for a film
miniature camera films, sheet films and plates. in this recommendation, It is a rating
It is believed that the method represents an or plate for use in connection with exposure
improvement over other commonly used proce- tables, exposure computers, exposure meters
dures, since it more fully satisfies the two and similar devices for obtaining properly
fundamental requirements of a system intended exposed negatives. The term “Exposure Index”
for international use : was chosen to avoid confusion with Speed.
(I) The method must give results which can Exposure Indexes are expressed in cube-root-
be applied directly and significantly to picture- of-2 steps, and may be calculated on either an
taking practice. arithmetic or a logarithmic scale. The arith-
(2) The method must be simple to operate metic numbers may be used directly with most
and must give unequivocal results. American-made exposure meters and exposure
The recommendation applies to individual computers. Similarly, the logarithmic scale
samples and to a product as a whole. The numbers may be used directly with many
sensitometric criterion upon which the method meters and computers of European manufacture.
* Loyd A. Jones: The evaluation of negative film speeds in terms of print quality. Journal of the Franklin Institute, p. 227,
297 and 497 (1939).
Loyd A. Jones and C. N. Nelson: A study of various sensitometric criteria of negative film speeds.
Journal of the
Optical Society of America, p. 30, 93 (1940).
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ISO/R 6 - 1954 (E)
IS0 Recommended Method for Determining
Photographic Speed and Exposure Index
obtain precise values of absolute Speed since,
1. Scope
in addition to making suitable negatives, many
1.1 This method of determining Speed and Expo-
prints must be made and a large panel of judges
sure Index applies to roll films, film packs,
must make a multitude of observations.
miniature camera films*, sheet films and plates
A practical method of measuring Speed has
intended for the making of monochromatic,
been found in the field of sensitometry.
The
continuous-tone negatives in pictorial photogra-
method is reasonably simple and rapid to
phy, exclusive of photography in the infrared.
operate, is reproducible, and gives results in
Process films and other graphic arts films and
close agreement with those obtained by the
plates, radiographic films, and aerial photogra-
laborious process of making picture negatives
phic films are excluded.
and prints.
2. Discussion of Problem and
Explanation of Terms
-
2.0
2.1 Concept of Speed for the Materials in this
Specification. The photographic Speed of the
negative materials referred to in 1.1 is to be
considered as inversely proportional to the
minimum exposure which must be incident upon
the negative material, from the scene element
of minimum brightness in which detail is visible,
in order that a print of as good quality can be
made from the resultant negative as from
DENSITY OF
-
negatives resulting from increased exposure of
BASE PLUS
I
FOG
*I I O
the same negative material. It is understood
Ë 2.0 1.0 O
that the word “visible” as used in the previous LOG EXPOSURE
sentence implies visibility of detail from the
camera position to a human observer having
FIG. 1
normal visual acuity and normal sensitivity to
brightness differences. It is also understood
2.2 Sensitometric Criterion of Speed. The ope-
that this concept of photographic Speed pre-
ration of this method is illustrated in Fig. 1.
supposes a scene of approximately normal
Essentially, it consists of plotting the density/
brightness contrast**.
log-exposure curve of a photographic material
In the field of negative-positive, black-and-
for a given set of conditions (exposure, develop-
white materials, this concept implies that Speed
ment, etc.). A log exposure range of 1.50,
can be measured by making the best possible
represented in the figure by the distance AB,
print from each of a series of negatives (which
is then moved along the horizontal axis from
differ only in the exposure given) and then
left to right until the slope (or gradient) of the
deciding by observation the minimum negative
curve at the low end of the range is 0.30 of
exposure that will lead to a print which is as
the average slope over the entire range. When
good in quality as any print made from the
the slope or tangent of
angle a is 0.30 of p
series of negatives. Speed measured by this
(i.e., the tangent of angle b), the point C, at
psychophysical method is called “absolute
the low end of the log exposure range, repre-
Speed”. In practice, it is found difficult to
sents the exposure value (E) from which the
Speed of the material is derived. A gradient
* Miniature camera films, of the negative-positive, black-
meter * simplifies the operation of locating
and-white type, are considered to be films not more than
point C.
35 mm in width used in still cameras for the making of
negatives which normally require enlargement in printing. Speed is then computed by use of the for-
** Such a normal scene may be characterized with suffi-
mula :
cient precision for the present purposes as one in which
Speed = 1/E
the ratio of the maximum to the minimum brightness is
128. With a typical camera, such as is used by the amateur,
.. .
of maximum to minimum illumination
this results in a ratio
* L. D. Clark: Gradient meter for use in photographic
on the negative material in the camera of 32. With the
use of coated lenses this average illumination ratio may sensitometry. PSA Journal, Photographic Science and
Technique, p. 87, 17B (1951).
be somewhat greater.
I
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lSO/R 6 - 1954 (E)
Adoption of this sensitometric criterion for Where films are used with exposure meters
determining Speed is basic to this recommenda- or exposure computers marked in logarithmic
tion. units, it may be desirable to express the Expo-
sure Indexes in logarithmic units. For this
Since several systems for determining Speed
purpose the logarithmic Exposure Indexes
are quite widely used throughout the world,
shown in Table 1 may be used as an alternative
Speed measured by using the IS0 Recommended
to the arithmetic Exposure Indexes. The for-
sensitometric criterion should be clearly marked
mula for obtaining the logarithmic values is
to avoid confusion. This may be accomplished
given in 3.2.1. The logarithmic indexes are
by using the prefix O. For example, if 1/E
referred to in degrees to distinguish them from
equals 200, the Speed would be written 0200.
the arithmetic indexes.
If, in addition to use of the IS0 Recommended
If Exposure Indexes have been obtained by
sensitometric criterion for calculating Speed,
following the details of one of these methods,
the IS0 Recommended testing technique (expo-
they shall be designated either “IS0 Recom-
sure, processing, etc.) is used, as described in
mended Arithmetic Exposure Index” or “IS0
these specifications, the Speed shall be known
Recommended Logarithmic Exposure Index”.
as “IS0 Recommended Speed.” For example,
A value would then be designated “IS0 Recom-
a sample of film exposed, processed, and eva-
mended Arithmetic Exposure Index 50” or
luated in strict accordance with the complete
“IS0 Recommended Logarithmic Exposure
method might give a value for Speed which
Index 28’”. If, in the presentation of these
would be written “IS0 Recommended Speed
data, insufficient space is available for use of
0200”. If, in the presentation of these data,
the full designation “IS0 Recommended”, as
insufficient space is available for the use of
for example in tabular material, sales or pro-
the full term “IS0 Recommended”, as in tabular
motional literature, the initials “ISO” may be
material, sales or promotional literature, the
used in substitution for the full designation.
initials “ISO” may be used in substitution of
The value for Exposure Index would then be
the full designation. The value of Speed would
written “IS0 Exposure Index 50” or “IS0
then be written “IS0 Speed 0200”.
Exposure Index 28 O”.
Should some factor be varied in the testing
technique (that is, in the production of the
density/log-exposure curve), the designation
3. Determination of IS0 Recommended
“IS0 Recommended” or the initials “ISO”
Speed and IS0 Recommended
must not be used. However, as stated above,
Exposure Index of a Specific Sample
if the sensitometric criterion has been used,
the Speed may still be written with the prefix O,
3.1 Determination of Speed
such as 0160.
3.1.1 Speed shall be computed by the use
of the formula:
2.3 Exposure Index. An Exposure Index is the
Speed = 1/E
rating of a film for use in connection with
exposure tables, exposure computers, and expo-
where E is the exposure (expressed in meter-
sure meters. Experience has shown that if
candle-seconds) corresponding to the point on
the IS0 Recommended Speed is divided by 4,
the density/log-exposure curve at which the
the numbers resulting will correspond to those
gradient is 0.3 times the average gradient for
now commonly used in exposure tables, expo-
a log exposure range of 1.50 of which E is
sure computers, and exposure meters employing
the minimum exposure.
arithmetic scales. The numbers so derived are
called “Arithmetic Exposure Indexes”.
3.1.2 Speed Scale. Values of Speed shall be
It should be remembered that Speed obtained
expressed arithmetically on a cube-root-of-2
by the use of these specifications relates to
scale. They shall be calculated as described
the minimum camera exposure which will lead
above but shall be rounded off to the nearest
to a print of as good a quality as can be obtained
cube-root-of-2 step as indicated by Table 1.
from negatives resulting from increased expo-
sure. In normal photographic work it is custom-
3.1.3 Speed shall be written with the prefix O.
ary to recommend a somewhat greater expo-
sure in order to allow for uncertainties in pro-
3.1.4 Speeds determined by the above for-
cessing, lighting conditions, camera operation,
mula are not intended for use with ordinary
etc. Exposure tables, computers, and exposure
exposure meters or exposure calculators.
meters applying Exposure Indexes are designed
to take into consideration this additional
exposure.
3.2 Determination of Exposure Index
Values of Arithmetic Exposure Index are
not written with the prefix O and, consequently, 3.2.1 Exposure Indexes for the types of
are readily distinguishable from values of Speed. photographic materials covered by this recom-
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lSO/R 6 - 1954 (E)
TABLE 1
Arithmetic
Logarithmic
For a log E Value
Speed Exposure Exposure
between *
Index
~ Index
(6.35 - 10) and (6.45- 10) 04000 1000 41"
- 10) and
(6.45 (6.55 - 10) 03200 800 40"
(6.55 - 10) and (6.65 - 10) 02500 650 39"
(6.65 - 10) and (6.75 - 10) 02000 500 38"
(6.75 - 10) and
(6.85 - 10) 01600 400 37"
(6.85 - 10) and (6.95 - 10) 01250 320 36"
(6.95 - 10) and (7.05 - 10)
01000 250 35"
(7.05 - 10) and (7.15 - 10) 0800 200 34"
(7.15 - 10) and
(7.25 - 10) 0650 160 33"
(7.25 - 10) and
(7.35 - 10) 0500 125 32"
(7.35 - 10) and (7.45 - 10) 0400 100 31"
- 10) and
(7.45 (7.55 - 10) 0320 80 30"
(7.55 - 10) and (7.65 - 10) 64 29"
0250
(7.65 - 10) and (7.75 - 10) 0200 50 2
...