ISO 517:2008

INTERNATIONAL ISO
STANDARD 517
Third edition
2008-03-15
Photography — Apertures and related
properties pertaining to photographic
lenses — Designations and
measurements
Photographie — Ouvertures et grandeurs associées relatives aux
objectifs photographiques — Désignations et mesurages

Reference number
©
ISO 2008
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ii © ISO 2008 – All rights reserved

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
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
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International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. 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.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 517 was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee SC 1,
Fundamental standards.
This third edition cancels and replaces the second edition (ISO 517:1996) which has undergone minor
technical revision.
INTERNATIONAL STANDARD ISO 517:2008(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 Terms and definitions
For the purposes of this document, the following terms and 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 of the photographic lens
f
limiting value of the image size h′ of a sharp imaged far-distant object h divided by its angular extension ω in
the object space i.e.
h′
− lim
f =
ω→ 0
tan(ω)
See Figure 1.
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
NOTE For photographic applications, the relative aperture is equivalent (within a 1/3 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 (2.5)
2.7
true f-number
unrounded standard f-number
See 3.2.2.
Key
1 lens under test
NOTE The object size h is positive, the image size h′ is negative and ω is positive.
Figure 1 — Focal length of a photographic lens (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, for
example 1:2,8.
Where preferable, the symbol f/ followed by the number value may be used, for example f/2,8.
3.2 Marking series
3.2.1 Standard series of f-number marking
The standard series of f-number marking shall be as follows:
0,5 – 0,7 – 1 (or 1,0) – 1,4 – 2 – 2,8 – 4 – 5,6 – 8 – 11 – 16 – 22 – 32 – 45 – 64 – 90 – 128.
NOTE When f-numbers are marked on a lens, points may be used as decimal signs instead of commas.
3.2.2 Standard f-number series
The standard f-number, or “whole stop” series shall be calculated according to the following formula:
m
f -number= 2
where m = −2, −1, 0, 1, 2, . (m is an integer).
2 © ISO 2008 – All rights reserved

3.2.3 f-number of the maximum relative aperture
The f-number of the maximum relative aperture, that is the smallest f-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 1:1,9 lens could be marked 1,9 − 2,8 − 4 − 5,6 − 8 etc., if it were believed that to mark it 1,9 − 2,0 − 2,8
− 4,0 − 5,6 etc., would confuse the markings at the 1,9 end of the scale.
3.2.4 Subdivisions of the standard f-number
Each standard f-number, or “whole stop”, division of scale markings may be divided into two or three
subdivisions in 1/2 or 1/3 steps of a stop calculated, for 1/2 subdivision, in a geometric series with first term
4 6
0,5 and factor 2 and for 1/3 subdivision in a geometric series with first term 0,5 and factor 2 .
Table 1 gives the calculated whole, half and third stops.
Table 1 — f-number series
Whole stop Half stop Third stop Whole stop Half stop Third stop
0,500 0,500 0,500 11,31 11,31 11,31
0,561  12,70
0,595 13,45
0,630  14,25
0,707 0,707 0,707 16,00 16,00 16,00
0,794  17,96
0,841 19,03
0,891  20,16
1,000 1,000 1,000 22,63 22,63 22,63
1,122 25,40
1,189 26,91
1,260 28,51
1,414 1,414 1,414 32,00 32,00 32,00
1,587 35,92
1,682 38,05
1,782 40,32
2,000 2,000 2,000 45,25 45,25 45,25
2,245 50,80
2,378 53,82
2,520 57,02
2,828 2,828 2,828 64,00 64,00 64,00
3,175 71,84
3,364 76,11
3,564 80,63
4,000 4,000 4,000 90,51 90,51 90,51
4,490 101,6
4,757 107,6
5,040 114,0
5,657 5,657 5,657 128,0 128,0 128,0
6,350
6,727
7,127
8,000 8,000 8,000
8,980
9,514
10,08
4 Tolerances of f-numbers for photographic lenses
The measured f-numbers shall equal the true f-number within the tolerances given in Table 2. If the full
aperture f-number is selected from the standard series, the tolerance shall be applied to the true f-number.
Table 2 — Tolerances of measured f-numbers
Marked f-number Tolerances
(to true f-number)
Full aperture ± 5 %
Smaller than f/5,6
+ 12 % − 11 % (± 1/3 stop)
f/5,6 and larger
+ 19 % − 16 % (± 1/2 stop)
5 Me
...