EOS Documentation Project

Flash Metering Pattern

by Julian Loke

Contents

• Revision History
• Comments

ETTL can be quite complicated, and largely undocumented. That is why I rely only on my own data to form my conclusions. This is the raison d'être for the EOS documentation project http://eosdoc.com

I devised the following test conditions. Use a 420EX mounted on an Elan 7E with 28-135 lens set to 135mm. The test target is a shaving mirror on a table at the end of a long corridor, about 10 meters / 30 feet away. Set M mode with 1/125 and f/32, set ISO to 100, CF8=1. IMPORTANT: Lens must be set to AF, and CF4 set to 0

Under these conditions, I get a "flash underexposure" warning if I FEL against any part of the target other than the direct specular reflection of the flash in the mirror (either with FEL or on my 420EX with a test exposure). If you try this for yourself and do not get a flash underexposure warning, change the ISO to a lower value (e.g. ISO 50, or 25) and try again.

With this test method, I can show that flash metering occurs at the single metering segment located under the active AF sensor. On my Elan 7E, this is simple, since metering segments are arranged in a rectangular 5x7 grid and each one of the seven AF sensors occupies one of the segments. See:
http://www.canon-sales.co.jp/7/product/point4/index-j.html
and
http://www.canon-sales.co.jp/7/tech/page/p03.html

If I switch the lens to MF, then flash registers at ANY of the 35 metering segments. This agrees with the Canon Letter on Donald Cohen's website.

I have not seen any other rigorous, easily repeatable tests for flash metering. Have you?

Revision History

0.1 2003Feb25 jul: draft; edits

Comments

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