How ETTLII Works

With many people buying/owning the 20D and the 1D Mark II, I thought it would be useful to post this:
Here is Chuck Westfall of Canon's post:
"E-TTL II is never linked to the active focusing point. In fact, that is one of the main differences between E-TTL II and the original version of E-TTL. There's a brief description of E-TTL II with sample images on Canon Inc.'s EOS-1D Mark II web site here.

Here's what we published in the EOS-1D Mark II White Paper document:

ADVANCED E-TTL II
New algorithm gives greater flash exposure control

For improved flash control using Canon EX series Speedlites, a new algorithm has been developed which enables superior E-TTL flash accuracy and reliability. In previous cameras, evaluative flash metering was based on the assumption that an autofocus point would cover the subject. When this is not the case, inaccurate flash exposures result. The EOS-1D Mark II’s evaluative flash metering is not dependent on the active AF point.

In the new algorithm, ambient light is measured when the shutter button is pressed. Next, a pre-flash is fired and the metering sensor takes readings at the central 17 metering zones. The ambient and pre-flash readings are compared. The metering areas having a small difference are selected as the flash exposure metering areas. (Areas with very big differences between ambient and pre-flash readings are excluded or down weighted because they are assumed to contain a highly reflective object or that the subject is not in that part of the frame. The algorithm avoids chronic underexposure problems in such situations.) These readings are weighted, averaged and compared with the ambient light reading, and the main flash output is then set and stored in memory. E-TTL II weights and averages the flash metering for the subject and all other objects at the same distance as the subject. Even if the subject’s position, reflectance or size changes, the flash output will not change radically. The flash exposure will be highly accurate and stable.

Most EF lenses provide distance information, and this data is also considered in determining if there is a highly reflective object, once again lessening the chance of underexposure.
***************

Additional info on E-TTL II:

In essence, distance information is not required for E-TTL II. But when it is available and the flash is direct, then it can be used as a reference to modify the flash exposure if necessary.

Additionally, the EOS-1D Mark II is provided with a new Custom Function (C.Fn 14-1) that allows photographers to select between evaluative and averaged flash metering in E-TTL II. Averaged flash metering may be preferable when using direct flash with lenses that don't have a built-in distance encoder, but we encourage Mark II owners to try both settings on C.Fn 14 to see which metering method they prefer.

E-TTL II is functional with all EF lenses, not just the ones with distance data. If you use a lens without distance data, the only thing you lose is the Mark II's ability to modify the flash exposure based on distance data. Everything else works the same. The Mark II *never* falls back to E-TTL.

There's never any "full-frame" flash metering in E-TTL or E-TTL II. In both cases, with EOS cameras that use a 45-point focusing system/21-zone metering sensor, all flash metering is carried out by the 17 metering segments within the Area AF ellipse shown in the viewfinder. Subject matter outside the ellipse is completely ignored in terms of flash exposure control.

C.Fn 14-0 on a Mark II allows E-TTL II flash metering to be subject-based, so it can use anywhere from 1 to 17 metering segments depending on the camera's analysis of the pre-flash information. This is not a spot meter reading, unless the camera determines that the subject is so small that it occupies only one of the 17 metering segments. Most subjects will cover a larger area than that. The big improvement here over the original E-TTL algorithm is that the size and location of the primary flash metering area can change dynamically according to the size and location of the subject. In the original E-TTL algorithm, the size, location and weighting of the primary flash metering area was linked to the active focusing point.

C.Fn 14-1 on a Mark II applies the E-TTL II flash metering algorithm equally to all 17 metering segments within the Area AF ellipse, so in comparison to 14-0, 14-1 is not subject-based.

As I've mentioned in previous posts, the setting of the focus mode switch on the lens has no bearing on E-TTL II flash metering, so C.Fn 14-0 and 14-1 are effective whether the lens is set for AF or manual focus.

***

There are two differences between original E-TTL and E-TTL II in this particular comparison:

1. E-TTL II will factor in distance information when it is available during direct flash operation, regardless of the C.Fn 14 or C.Fn 4 setting. Standard E-TTL does not use distance info.

2. When C.Fn 14-1 is active on the Mark II, E-TTL II flashmetering is averaged for all 17 metering segments regardless of the C.Fn 4 setting or the focus mode set on the lens. This gives the photographer more flexibility in setting up the camera according to their personal preferences. Original E-TTL can't average its flashmetering unless specific camera settings are used."

---------------
Chuck Westfall
Director/Technical Marketing Dept.
Camera Division/Canon U.S.A., Inc.

The original thread is here: http://www.robgalbrait​h.com …iew=&sb=5&o=&fp​art=1&vc=1

There is also some info here: http://photonotes.org/​articles/eos-flash/#ettlii
Regards,
Scott

And so after 3 years, it is time to Bump this VERY important post again.

That is because :
a. The mentioned Rob Galbraith Forum link is un-available, and
b. That now there's a confirmation to this, in Canon CPN site:
http://cpn.canon-europe.com …ank/flash/flash​_basics.do

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Funny how they say reflective areas are downplayed or ignored to avoid underexposure, yet underexposure is exactly what does happen in real life.

roli_bark wrote in post #4055142
And so after 3 years, it is time to Bump this VERY important post again.

That is because :
a. The mentioned Rob Galbraith Forum link is un-available, and
b. That now there's a confirmation to this, in Canon CPN site:
http://cpn.canon-europe.com …ank/flash/flash​_basics.do

I'm not sure what the purpose of this post is. Is it just to confirm what Scott already posted because I didn't see anything new which was not already mentioned in the original post, unless I missed it?

cdifoto wrote in post #4055168
Funny how they say reflective areas are downplayed or ignored to avoid underexposure, yet underexposure is exactly what does happen in real life.

It all depends on where the reflective area is. If it's right beside the subject so that the camera has a hard time distinguishing the two, then you will definitely get underexposure. However, if there is a separation between the subject and the reflective area, you won't get any underexposure if the flash metering is done in Evaluative mode. You see this at work in this thread:

https://photography-on-the.net/forum/showthre​ad.php?t=108846

PacAce wrote in post #4055307
I'm not sure what the purpose of this post is. Is it just to confirm what Scott already posted because I didn't see anything new which was not already mentioned in the original post, unless I missed it?

Sorry PacAce, but I was trying to find the mentioned original by Chuck Westfall to re-confirm my understanding on ETTL-II.

Then I found this on Canon CPN. Altough not new to you, I find some new facts - for example the claim [In Canon FlashWork site] that in the Ambient metering phase only segments linked to the AF point participate, are nullified here !.

That was NEW & important for me enough to clear MY confusion.

To be more specific:

1. Scott Berger above quoting Chuch Westfall:
" ... In the new algorithm, ambient light is measured when the shutter button is pressed. Next, a pre-flash is fired and the metering sensor takes readings at the central 17 metering zones..."

2. Canon FlashWork site:
http://web.canon.jp …tl2/technology/​index.html
" ...Shutter button pressed halfway. Autofocusing and evaluative metering (with the multizone sensor linked to the focusing point) are executed simultaneously. The ambient light is thus metered.... Shutter button pressed completely....etc ...

3. From CPN:
http://cpn.canon-europe.com …ank/flash/flash​_basics.do
"... E-TTL autoflash works on the assumption that the subject will be covered by an autofocus (AF) point. However, an AF point does not always cover the area of the subject where you want the flash exposure to be correct. The E-TTL II autoflash system overcomes this problem. When you press the shutter button, the ambient light is metered by each sensor. Then a pre-flash fires and this is also metered by all the sensors. ..."

The latter (3) is the MOST precise, it clears the confusion (highlighted in RED) caused by (2), and elaborates more precisely what (1) describes independently of Camera type

roli_bark wrote in post #4055349
Sorry PacAce, but I was trying to find the mentioned original by Chuck Westfall to re-confirm my understanding on ETTL-II.

Then I found this on Canon CPN. Altough not new to you, I find some new facts - for example the claim [In Canon FlashWork site] that in the Ambient metering phase only segments linked to the AF point participate, are nullified here !.

That was NEW & important for me enough to clear MY confusion.

OK, I see. Thanks for the clarification.

BTW, can you point me to the sections in the Canon Flash Works sites you were referring to? I looked but couldn't find any reference to ambient metering and AF points only anywhere although I do admit that I only looked in the obvious places. Thanks.

PacAce wrote in post #4055437
OK, I see. Thanks for the clarification.

BTW, can you point me to the sections in the Canon Flash Works sites you were referring to? I looked but couldn't find any reference to ambient metering and AF points only anywhere although I do admit that I only looked in the obvious places. Thanks.

See my specific follow-up above ...

roli_bark wrote in post #4055407
To be more specific:

1. Scott Berger above quoting Chuch Westfall:
" ... In the new algorithm, ambient light is measured when the shutter button is pressed. Next, a pre-flash is fired and the metering sensor takes readings at the central 17 metering zones..."

2. Canon FlashWork site:
http://web.canon.jp …tl2/technology/​index.html
" ...Shutter button pressed halfway. Autofocusing and evaluative metering (with the multizone sensor linked to the focusing point) are executed simultaneously. The ambient light is thus metered.... Shutter button pressed completely....etc ...

3. From CPN:
http://cpn.canon-europe.com …ank/flash/flash​_basics.do
"... E-TTL autoflash works on the assumption that the subject will be covered by an autofocus (AF) point. However, an AF point does not always cover the area of the subject where you want the flash exposure to be correct. The E-TTL II autoflash system overcomes this problem. When you press the shutter button, the ambient light is metered by each sensor. Then a pre-flash fires and this is also metered by all the sensors. ..."

The latter (3) is the MOST precise, it clears the confusion (highlighted in RED) caused by (2), and elaborates more precisely what (1) describes independently of Camera type

All the statements above (1, 2, and 3) are consistent with each other. The AF points referred to in #2 is in reference to the ambient light metering which has always been true when Evaluative metering is used.

PacAce wrote in post #4056770
All the statements above (1, 2, and 3) are consistent with each other. The AF points referred to in #2 is in reference to the ambient light metering which has always been true when Evaluative metering is used.

Here's the contradiction, as I see it:
(2) says:
"... Shutter button pressed halfway ... evaluative metering ... with the multizone sensor linked to the focusing point ..."

(3) says:
"...When you press the shutter button, the ambient light is metered by each sensor..."

Questions:
a. What happens to ETTL-II when I shoot Partial, Spot, or CW, and my AF pointed to the extreme left ?

b. Can I conclude that (3) still means "each sensor", but with more weight assigned to the one linked to the AF points ?

c. Is this what 'Ambient Evaluative Metering' is all about ? [assigning more weight to segment linked to the AF point], or comparing the scene to many other pre-recorded scenes stored in memory to get optimal ambient exposure ?

roli_bark wrote in post #4061101
Here's the contradiction, as I see it:
(2) says:
"... Shutter button pressed halfway ... evaluative metering ... with the multizone sensor linked to the focusing point ..."

(3) says:
"...When you press the shutter button, the ambient light is metered by each sensor..."

Questions:
a. What happens to ETTL-II when I shoot Partial, Spot, or CW, and my AF pointed to the extreme left ?

b. Can I conclude that (3) still means "each sensor", but with more weight assigned to the one linked to the AF points ?

c. Is this what 'Ambient Evaluative Metering' is all about ? [assigning more weight to segment linked to the AF point], or comparing the scene to many other pre-recorded scenes stored in memory to get optimal ambient exposure ?

First let me clarify #2 and #3, then I'll answer your questions.
#2 basically describes what Evaluative ambient light metering is. It uses all the metering sensors with bias given to the AF point s(this is what the "sensor linked to the focusing point" is referring to). #3 says the same thing without mentioning anything about the AF points. But it doesn't exclude them either. Hence, #3 is a subset of #2.

Re your question:

a) How your ambient light is actually metered depends on what camera you are using. The 1D series cameras always switch to Evaluative ambient metering when a flash is in use no matter what the camera is actually set to. This curious phenomenon was observed by a fellow POTNer last year and he posted a question here. After confirming his observation myself with my 1DmkII, I wrote to Chuck Westfall about this and his response was posted in his Tech Tips article in the June 2006 issue of Digital Journalist magazine. If you're interested in reading it, click on the link below and go to the last question:

http://www.digitaljour​nalist.org/issue0606/t​ech-tips.html

With a non-1D series camera, ambient lighting would be metered via whatever metering mode has been selected by the photographer.

b) I think what's been confusing you is Canon's mentioning of the AF points in reference to the ambient light metering. My guess is that they are assuming that people are using Evaluative metering by default or they're talking only about the 1D series cameras. When you use spot, partial or CWA metering, AF points do not come into play. They only do in Evaluative metering mode.

c) Yes.

Thanks Leo for the info, especially for the link about a 1D series type swtiching automatically to 'Ambient Evaluative Metering' when a Flash is sensed.

Ah-ha! My dad is a wedding photographer (actually has some Angie's List reviews), and he always grumbled and growled about 1D series. This was a few years ago, so he was obviously talking over my head about tech stuff. I'll had to point some of this info out to him (like Canon admitting there was a bit of a problem), and see if he feels any better about his 1D.