EOS Documentation Project

Julian's Autofocus Sensitivity Test

by Julian Loke

Contents

• Introduction
• Method
• AF Sensitivity Test Target
• Revision History
• Comments

Introduction

The issue of low-light autofocus performance has been the subject of much recent debate amongst EOS camera users. Initial reports from early adopters of the EOS D30 digital camera and the EOS Elan 7E / 7 / 30 indicated that the low light AF performance was annoyingly worse than the official Canon specification.

Some reports compare cameras at different market points, and attempt to draw a conclusion based on the speed of autofocus acquisition, or on the tracking of a moving subject. Sometimes, autofocus is assisted with a built-in lamp on the camera, or from a patterned beam emitted from an accessory flash device.

Autofocus ability will not only depend on the brightness of the scene, but also on the pattern presented to the camera. It is even possible to fool the autofocus with patterns that cannot be focussed at all!

Method

This document presents an AF sensitivity target that can be used for objective measurement of the limit of autofocus sensitivity in EOS cameras in low light.

The target consists of a web page with a series of rows of alternating light and dark patches. The camera should be able to autofocus onto the transition, but it will fail if the difference in brightness is too small for the AF sensor to detect.

By metering the black/white/limit patches, it should be possible to compare the autofocus performance from different cameras. Perhaps those duelling EOS D30 and EOS D60 users, or the Elan 7E detractors, can finally settle their longstanding arguments. Or perhaps start new ones!

AF Sensitivity Test Target

The test target is the sharp transition between the light patch on the left and the black patch on the right. Scroll up and down the page and test if the camera is able to lock onto the transition. Of course, the AF assist beam has to be disabled to make comparisons meaningful between different cameras.

Record the spot or partial meter reading for BLACK, and the reading for WHITE, and the patch number, as well as the meter reading over the patch.

For instance, My Elan 7E is able to AF down to patch number 65, with some hunting.
. Black = 1"@f/2.8, EV 3
. White= 1"@f/22, EV 9
. Patch 65= 1"@f/8, EV 6

Julian's Ev calculator

You are welcome to enter the results from testing your own camera in the comments at the end of this page.

Please try again with a different browser.

Revision History

v0.4b2002Sep04 jul: minor reformat
v0.4 2002Apr26 jul - more steps
v0.3 2002Apr20 jul - NS6.2 compat, 1/2 Tv steps, 0.5EV steps
v0.2 2002Apr19 jul - Added EV calculator
v0.1 2002Apr18 jul - original

Comments

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