ISO/TS 20490:2024

Technical
Specification
ISO/TS 20490
First edition
Measuring autofocus repeatability
2024-03
of sharpness and latency
Mesure de la répétabilité de la netteté et de la latence de
l’autofocus
Reference number
© ISO 2024
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions and abbreviated terms . 1
4 Test description . 2
4.1 General .2
4.2 Test device settings .2
4.3 Environmental conditions .2
4.4 Apparatus and hardware .2
4.5 Test distance .4
4.6 Test charts .4
4.6.1 General .4
4.6.2 Recommended test charts .5
4.7 Test conditions .5
4.8 Sharpness measurement .6
5 Autofocus measurement . 6
5.1 General .6
5.2 Camera test position and distances .6
5.3 Capture test sequence .6
5.3.1 Test sequence for continuous autofocus (AF-C) .6
5.3.2 Test sequence for single autofocus (AF-S) .7
6 Presentation of results . 10
6.1 Result figure .10
6.2 Reporting the results .11
Annex A (informative) Test setup and chart details .12
Annex B (informative) Test software and algorithm details: sharpness estimation algorithm . 14
Annex C (informative) User reaction time test results during photo capture . 17
Annex D (informative) Correlation between subjective sharpness and sharpness score .20
Bibliography .21

iii
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 non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 42, Photography.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
This document is focusing on measuring the repeatability of the AF latency and the sharpness of the
captured images. ISO 15781 specifies how to measure and report the shooting time lag, shutter release time
lag, shooting rate and start-up time lag for digital still cameras. This document focuses on combining the
autofocus latency with measured sharpness of the captured photos, making it more comprehensive test
procedure for evaluating autofocus systems.
This document widens the options for usable test charts from high contrast digitally created charts to
natural images and to other test charts and even 3D scenes, challenging the autofocus systems. It also allows
measurements to be carried out in variable lighting conditions, and in presence of handshake, challenging
the AF system further.
ISO 15781 is mainly focusing on traditional single autofocus solutions actuated by half pressing physical
shutter button, widely used with SLR cameras. However, this document can be applied to continuous AF
systems, commonly used in mobile camera devices, as well as to single autofocus systems.
This document provides procedures and methods to measure and report the autofocus (AF) repeatability
of sharpness and latency of a digital still camera. The data gathered is useful when comparing camera
devices with sufficiently similar autofocus solutions and it helps with further investigations into a camera’s
autofocus repeatability performance.
The terminology is defined within this document along with describing the test charts, the setup, the
methods, the performance metrics and analysis methodology to assess and report on the autofocus
repeatability of a camera device. This document covers the test setups, the process, what pictures to capture
and the metrics to calculate.
A great camera system should be capable to deliver repeatably sharp images within acceptable and
repeatable latency, making the characterization of the AF system very important.

v
Technical Specification ISO/TS 20490:2024(en)
Measuring autofocus repeatability of sharpness and latency
1 Scope
This document is focused on measuring the autofocus (AF) repeatability of sharpness and latency, meaning
camera system’s capability to produce sharp images within certain time frame. The scope of document is
limited to testing autofocus sharpness and latency repeatability with stationary charts only as testing with
moving charts is not covered.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 15781, Photography — Digital still cameras — Measuring shooting time lag, shutter release time lag,
shooting rate, and start-up time lag
3 Terms and definitions and abbreviated terms
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
autoexposure
AE
system to automatically adjust the exposure parameters such as gain, exposure time and aperture
3.2
autofocus
AF
focusing system which can automatically control the optical system in a camera to bring a subject into focus
3.3
continuous autofocus
AF-C
autofocus system continuously keeping subject in focus
3.4
field of view
FoV
extent of the observable world that is seen (solid angle through optics to sensor) at any given moment by an
imaging system i.e. camera
3.5
depth of field
DoF
distance between the nearest and the furthest objects that are in acceptably sharp focus in an image
captured with a camera
3.6
spatial frequency response
SFR
relative amplitude response of an imaging system as a function of input spatial frequency
3.7
single autofocus
AF-S
focusing system which focuses on the selected target once, often activated by pressing camera button
halfway down, and keeps the selected focus until focused again
4 Test description
4.1 General
The measurement shall be carried out using output images from a digital still camera with which the test is
conducted.
The following measurement conditions should be used as nominal conditions when measuring the autofocus
repeatability of a digital still camera. If it is not possible to achieve these conditions, the actual capture
conditions shall be listed along with the reported results.
Target of the study is to measure if the devices in question are capable to focus and produce sharp images
within a certain timeframe which is based on measurements of human reaction time in photographic
situation per Annex C.
4.2 Test device settings
Cameras are to be tested in default out of the box settings. If testing is done with something else than out of
the box settings, those settings should be mentioned in the report.
4.3 Environmental conditions
The measurements shall be carried out in the following environment unless otherwise stated:
— temperature: 23 °C ± 3 °C.
4.4 Apparatus and hardware
The test setup consists of several components: close distance and far distance test charts, illumination setup
for both charts, actuated holder to move the close distance chart, timing LED panel, and computer system
to control the timing of the image capture and peripheral devices like the close distance chart actuation and
LED timing panel or timer.
In this document the test setups and recommendations are assuming usage of reflective test charts. In
special situations also transmissive test chart can be used, but particular care needs to be applied when
using transmissive charts. For example, the recommended lux conditions should be in-line with the used
panel brightness. The light flux from the chart shall be diffused and shall not include any specular component.
The camera holder should allow the centre of the camera optical axis to be aligned along a line perpendicular
to the chart in such a way that the optical axis is perpendicular for both close distance and far distance charts.
The close distance chart actuation stage shall allow the chart to be removed or inserted into the field of view
of the camera within 0,1 s or less, as required in ISO 15781.
Figure 1 shows the top view of the example test setup. The close distance and far distance charts are aligned
orthogonally to the camera. Baffles can be used to block light traveling directly from the light source to the
camera lens and minimize light being cast outside the 18 % grey area.

Key
A line A: Light rays from the edges of the lamps are shown intersection of which the 18 % gray background edges. ─
─ ─ ─
B line B: Camera HFoV ----
C line C: Camera optical axes ─ - ─ - ─ - ─
D line D: Light rays from close distance chart illuminators ─ ∙ ─ ∙ ─ ∙ ─
E line E: Light intensity pattern from the lamp is pointing parallel to the direction of this line. The line intersects the
target at 45 degrees - ∙∙ - ∙∙ - ∙∙ -
F close distance chart
G close distance chart actuator stage
H far distance chart
I LED timer
J camera under test
K close distance chart illuminants
L far distance chart illuminants
Figure 1 — Top view of example test setup

Key
A camera under test
B close distance chart
C far distance chart
Figure 2 — Side View of Test Setup. The close distance chart is moved in and out of the field of view
4.5 Test distance
For test distances, the following chart distances are recommended.
The close distance chart distance is set to 10 times the lens 35 mm equivalent focal length. The far distance
chart distance is set to 7 times the close distance chart distance. If the 10 times focal length close distance
cannot be achieved, then the closest possible focusing distance should be used.
For example, if the closest focusing distance for camera with 24 mm lens is 24 cm, the far distance chart
distance is 168 cm.
If recommended distances cannot be achieved, the actual used distances shall be clearly stated in the results.
4.6 Test charts
4.6.1 General
Test chart content can have a significant impact for autofocus repeatability and latency. Because of this, it
can be very useful to test the autofocus with different kinds of test charts. Using the algorithm provided in
Annex B, tests can be conducted with multiple types of test chart contents including natural or artificial

images, low or high contrast content, or even 3D scenes, assuming reasonable image information content
(entropy).
This test procedure is also applicable by using other proven sharpness charts with accompanying metrics,
for example spatial frequency response (SFR) or texture charts described in ISO 12233 and ISO/TS 19567-2
using the specified algorithms.
4.6.2 Recommended test charts
This test procedure can be conducted with several different test charts, including natural or artificial
images, low or high contrast content or 3D scenes. Including a photo realistic mannequin into the test
scene can reveal interesting properties of autofocus (AF) systems using face information in supporting the
AF decision making. However, when using 3D targets, it is important to consider the depth of field of the
camera system in question and ensure that the camera is correctly focused to the same depth plane on each
repetition and in case of camera comparison, each device is focused on the same depth plane. Due to the
image content impact on autofocus performance, devices shall be compared using similar test charts or 3D
scenes. There are examples of recommended test charts in the Annex A.
4.7 Test conditions
Illumination intensity and spectrum should remain the same for close distance and far distance charts
to minimize the autoexposure (AE) and AWB convergence time, and their overall impact to the measured
latency. Close distance and far distance charts should be illuminated at an illumination level within
±0,3 exposure values, aligned with the ISO 15781.
It’s recommended to carry out measurements in both bright and low light conditions using the testing
process described. Recommended test illuminations are 10 lx and 1 000 lx, but more conditions can be
included to gain further insight into the autofocus system under test.
Handshake can have an effect on autofocus behaviour. Therefore, it is allowed to incorporate handshake into
the test procedure where it serves the specific test needs. Handshake can be added for example by attaching
the capture device to a handshake simulator and using a handshake profile as described in the 2019 IS&T
International symposium on Electronic Imaging Science and Technology publication “Issues reproducing
[1]
handshake on mobile phone cameras” . However, note that this document is designed for autofocus
repeatability and latency measurement, not for evaluating the stabilization capability of the test device.
Table 1 provides the lighting and illumination requirements. No direct light should reach the lens from light
source within or outside the field of view of the camera. Target is to avoid having flare and/or ghost images
impacting the results.
Testing should take place in both high and low illumination conditions as outlined in Table 1. Ideally,
the illumination level for bright image should be high enough to reach the base ISO speed of the camera
in question. Colour temperature should reflect the real-life lighting conditions, meaning that bright light
testing should take place in higher colour temperature, and low light testing at low colour temperature.
Table 1 — Lighting and illumination requirements
Bright: 5 500 K ± 1 000 K.
Colour tempera-
ture
Low: 3 000 K ± 1 000 K.
Flicker The light sources are to not cause flicker.
It’s recommended to do measurements with at least two different light
Incident lux light
levels (10 lx and 1 000 lx) to ascertain focus repeatability performance
level
under low light and bright light conditions.
Incident lux measurements are to be made at the corner loca
...