I recently came across the EOS flash FAQ on Bob Atkin's web site, and was delighted to be getting solid info for the first time. I was especially excited about the explanation of exactly how the flash exposure is calculated. Dim light, flash exposes "subject", background exposed by speed/aperture; bright light, everything with speed/aperture, flash stepped down as much as 1.5 stops for fill-in. Add in the flash compensation, and you've got a lot of flexibility and power.

Then I came to the conclusion of this section, which read: "BTW, note that I am only speaking of ATTL and TTL here, [not] ETTL [for which] arbitrary flash-fill ratio does not apply." But earlier in the article, it said that the ElanII almost always uses ETTL. So, as I understand it, the article gives a specific, detailed explanation of the flash algorithm (just the kind of info I've been wanting for a long time), and then says it almost never applies.

What gives?

-- Ted Kostek (kostek@ecn.purdue.edu), November 30, 2001

Answers

Unfortunately, the EOS Flash FAQ is all we have, and it does not answer all the mysteries of ETTL logic. In bright, front lit or side lit fill flash situations, it generally performs as advertised, and reduces fill ratios by as much as 1.5 stops. The question is, in heavy backlit situations where the evaluative meter system could underexpose the main subject, is ETTL smarter than TTL, and know not to reduce fill ratios? I have found that it is smarter, but I might still add ½ to 1 stop of FEC, just to make sure.

I recommend that you test this yourself using slide film and careful notes for each exposure. I should retest my 550EX on the EOS 3, which can turn off fill flash ratio reduction, but I have not yet gotten around to it.

-- Kenneth Katz (socks@bestweb.net), November 30, 2001.

Auto reduction of fill flash in higher ambient light levels still happens. It just doesn't happen in a completely linear fasion as the ambient light levels go up. It actually compares the contrast areas of the metered scene and makes a determination about how much fill flash reduction it should apply. Canon states that with E-TTL as much as 2 stops of auto reducton takes place. Canon has never given clear answers to how much auto reduction applies to a particular situation or how that amount is determined.

As a general rule, E-TTL follows something close to the regular auto fill flash reduction used in TTL and A-TTL. But you never know for sure how much it is compensating, and that's always the problem of any kind of fancy metering scheme. Sometimes turning the camera to center weighted or partial area metering and disabling E-TTL, and then doing your own compensation, is a better way to meter. Unfortunately the 380EX doesn't give you that option without adding the HSA-2 under it.

-- Jim Strutz (j.strutz@gci.net), November 30, 2001.