ISO 517:1996
(Main)Photography — Apertures and related properties pertaining to photographic lenses — Designations and measurements
Photography — Apertures and related properties pertaining to photographic lenses — Designations and measurements
Pertains to apertures and related properties of photographic lenses affecting the illuminance at the centre of the image. Does apply only to lenses focused on objects at infinity; that is, at least 50 times the focal length of the lens.
Photographie — Ouvertures et grandeurs associées relatives aux objectifs photographiques — Désignations et mesurages
General Information
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Standards Content (Sample)
INTERNATIONAL IS0
STANDARD 517
Second edition
1996-12-15
Photography - Apertures and related
properties pertaining to photographic
lenses - Designations and measurements
Photographie - Ouvertures et grandeurs associ6es rela ties aux objectifs
photographiques - Dksigna tions et mesurages
Reference number
IS0 517:1996(E)
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IS0 517:1996(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
technical committees. Each member body interested in a subject for
which a technical committee has been established has the right to be
represented on that committee. International organizations, governmental
and non-governmental, in liaison with ISO, also take part in the work. IS0
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting
a vote.
International Standard IS0 517 was prepared by Technical Committee
lSO/TC 42, Photography.
This second edition cancels and replaces the first edition (IS0 517:1973),
of which it constitutes a technical revision.
0 IS0 1996
All rights reserved. Unless otherwise specified, no part of this publication may be
reproduced or utilized in any form or by any means, electronic or mechanical, including
photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-121
1 Geneve 20 l Switzerland
Printed in Switzerland
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INTERNATIONAL STANDARD @ IS0 IS0 517:1996(E)
Photography - Apertures and related properties pertaining
to photographic lenses - Designations and measurements
1 Scope
This International Standard pertains to apertures and related properties of photographic lenses affecting the
illuminance at the centre of the image.
This International Standard specifies aperture markings for all types of lenses used in still cameras, and gives
tolerances for the stop numbers. It also defines aperture stop, entrance pupil, focal length, relative aperture and
stop numbers, and gives methods for their measurement or determination.
This International Standard applies only to lenses focused on objects at infinity; that is, at least 50 times the focal
length of the lens.
2 Definitions
For the purposes of this International Standard, the following definitions apply.
2.1 aperture stop: Physical stop that limits the cross-section of the light beam that can pass through the lens to
reach the centre of the on-axis image.
2.2 entrance pupil: Image of the aperture stop as viewed from a point in the object space on its optical axis (the
image of the aperture stop formed by the front elements of the lens).
2.3 exit pupil: Image of the aperture stop as viewed from a point in the image plane of the lens and on its optical
axis (the image of the aperture stop formed by the rear elements of the lens).
2.4 focal length (f) of the photographic lens: Limiting value of the image size h’ of a sharp imaged far-distant
object h divided by its angular extension o in the object space (see figure I), i.e.
.
-
h’
Irm
-
f
- cc,+o tan(w)
2.5 relative aperture of a photographic lens: Twice the numerical aperture where the numerical aperture is the
sine of the semi-angle subtended by the exit pupil at the focal plane. For photographic applications, the relative
aperture is equivalent (within a l/3 stop) to the ratio of the diameter of the entrance pupil to the focal length.
2.6 f-number; stop number: The reciprocal value of the relative aperture (see 2.5).
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IS0 517:1996(E)
Lens under test
/--
The object size h is positive, the image size h’ is negative and CI) is positive.
NOTE -
Figure I - Focal length of a photographic lens (see 2.4)
3 Aperture markings
3.1 Designations
The relative aperture of a lens shall be designated by 1: followed by the numerical value of f-number, as in the
example 1:2,8.
Where preferable, the symbolfl followed by the number value may be used, for examplef/2,8.
3.2 Marking series
3.21 Standard series of f-number marking
The standard series off-number marking shall be as follows:
0,5-0,7-l (or l,O)-1,4-2-2,8-4-5,6-8-11 -16-22-32-45-64-90-128.
NOTE - Whenf-numbers are marked on a lens, points may be used as decimal signs instead of commas.
32.2 Standardf-number series
The standardf-number, or “whole stop” series shall be calculated according to the following formula:
m
f-number = 2 2
integer).
where m = -2, -1, 0, 1, 2, . . . (m is an
3.2.3 f-number of the maximum relative aperture
Thef-number of the maximum relative aperture, that is the smallestf-number pertaining to the given lens, need not
be selected from the standard series, but shall be followed by the series, beginning with the next larger number
whenever practical, and progressing as far as required in the individual application.
EXAMPLE
A 1 :I ,9 lens may be marked I,9 - 2,8 - 4 - 5,6 - 8 etc., if it were believed that to mark it I,9 - 2,0 - 2,8 - 4,0 - 5,6
etc., would confuse the markings at the I,9 end of the scale.
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IS0 517:1996(E)
@ IS0
32.4 Subdivisions of the standardf-number
division of scale markings may be divided into two or three subdivisions
Each standard f-number, or “whole stop ”,
in l/2 or l/3 steps of a stop calculated, for l/2 subdivision, in a geometric series with first term 0,5 and factor E
and for l/3 subdivision in a geometric series with first term 0,5 and factor 6fi.
Table 1 gives the calculated whole, half and third stops.
Table 1 -f- number series
Whole stop Half stop Third stop
0,500 0,500 0,500
0,561
0,630
0,707 0,707 0,707
0,794
0,891
1,000 1,000 1,000
1,189 1,122
1,260
1,414 1,414
1,414
1,682 1,587
1,782
2,000 2,000 2,000
2,378 2,245
2,520
2,828 2,828
2,828
3,364
3,175
3,564
4,000 4,000 4,000
4,757
4,490
5,040
5,657 5,657 5,657
6,727 6,350
7,127
8,000 8,000 8,000
9,514 8,980
IO,08
II,31 II,31
II,31
13,45 12,70
14,25
16,00 16,00 16,00
19,03 17,96
20,16
22,63
22,63 22,63
26,91 25,40
28,51
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IS0 517:1996(E)
Table 1 (concluded)
32,00 32,00 32,00
38,05 35,92
40,32
45,25 45,25 45,25
53,82 50,80
57,02
64,00 64,00 64,00
76,ll 71,84
80,63
go,51 go,51
go,51
107,6 101,6
114,0
128,0 128,0 128,0
4 Tolerances off-numbers for photographic lenses
The measuredf-numbers shall equal the truef-number within the tolerances given in table 2.
Table 2 - Tolerances of measuredf-numbers
\
,
Markedf-number Tolerances (to truef-number)
+ 5 %
Full aperture
Smallerthanfl5,6 +12% -11 % (+1/3stop)
f/5,6 and larger +19% -16 % (+1/2stop)
\
If the full aperturef-number is selected from the standard series, the tolerance shall be applied to the truef-number.
5 Methods for measuring effective entrance pupil and focal length
5.1 General
There are a number of possible procedures for measuring the effective entrance pupil and focal length of a
photographic lens, which may be used if the measuring errors are within the permissible tolerances. As examples,
two of the methods most frequently used for measuring the entrance pupil and two methods for measuring the
f
focal length are given in 5.2.
5.2 Measurement methods
52.1 Effective entrance pupil
5.2.1.1 Method 1 for measuring the diameter of the effective pupil of a photographic lens
5.2.1.1.1 Principle
Method 1 uses a direct measurement of the entrance pupil diameter.
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@ IS0 IS0 517:1996(E)
5.2.1 .I .2 Apparatus
a) Travelling compo
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