Canon EOS dSLRs - Evaluative metering revealed

[Doug Kerr]

As a result of my inquiry in another forum about exactly how Evaluative metering works with AI Servo AF on Canon EOS dSLRs , I received a wonderful, thorough discussion of the issues from Chuck Westfall of Canon USA. Since my original inquiry was also posted here, I posted in that thread a link to Chuck's response, and made a few comments.

But I think what I learned (it may be old news to many of you) is so important I thought it deserved its own outing in a separate thread.

Here is Chuck's essay:

http://www.prophotohome.com/forum/ca...tml#post469325

I was startled to learn that, even if several AF points "light up", this merely indicates which points on the scene lie within the depth of field surrounding the distance at the one point that is used as the basis of AF (I of course realized that only one AF detector actually controlled focus), and it is only the area around that "principal" point (and we are not shown which one that is) that is given "emphasis" (whatever that is) by the Evaluative metering system. (I had always thought that the areas around all "lit" AF points were given "emphasis".)

I am quite certain that in my 300D days, this was not the impression I was given by Canon's explanations of the Evaluative metering system (although the details were never completely clear). Of course I may have then jumped to unwarranted conclusions.

The inference I drew was that the AF system, based on distance consideration and other things, decided what the "extent" of the subject was, and arranged for emphasis to be given to the areas around all AF points lying in that extent - a "subject-wide" intelligent metering algorithm.

Perhaps the doctrine has changed (although Chuck suggests not).

I also think I see in Chuck's remarks (as well as by the language in the 40D manual) that Canon today has a more modest view of when Evaluative metering is most appropriate than at an earlier time (when it seemed to be the answer to everybody's prayer for exposure metering).

The pivotal point is that if we have Evaluative metering in effect with Automatic selection of AF points ("all AF" points available), we cannot be sure which AF point (of all those that are lit) is the one (and the only one) whose surrounding area is given "emphasis" by the Evaluative metering system (and of course we still just barely know what that really means).

Now, here's the way this impacts my original reason for inquiring. I had been feeling sad that, if I preselect a single AF point, in order to get predictable focus, I was somehow missing the "intelligence" of the Evaluative metering system in discerning which AF points lay in areas warranting "emphasis" in metering and considering them all in making an exposure decision.

Well, it turns out that, had I used Automatic AF point selection, but managed (perhaps by good luck) to get the "principal" AF point to be on the subject, only the area around that AF point would have been given emphasis in metering anyway.

And I guess now I see why so many people have said, "Evaluative? Don't bother. I use Partial or AWB." Oldies but goodies.

[Doug Kerr]

Evidently the link I gave to access Chuck Westfall's post on ProPhto Home doesn't work for some or all our members.

Accordingly, I transcribe his message here. Again, this is taken from ProPhto Home.
*************
Hi, Doug:

The EOS 40D is consistent with other EOS Digital SLRs in terms of its evaluative metering algorithms, so what I'm about to say here applies just as well to other models like the 50D and the 5d Mark II, etc.

First, Evaluative metering is always linked to a single focusing point. This is true even when multiple focusing points light up simultaneously, as might be the case with a large subject when the camera is set for the combination of One-Shot AF or AI Focus AF and Automatic Focusing Point Selection. The active focusing points do not light up when the camera is set for the combination of AI Servo AF and Automatic Focusing Point Selection and the camera is focusing.

Speaking about One-Shot AF and AI Focus AF with stationary subjects: If only one focusing point lights up, then that one is the active focusing point. If more than one light up, it means that those points are all within the depth of focus as determined by the AF system, but the camera's AF detection algorithm selects the metering segment surrounding one of those points as the primary segment for the evaluative exposure measurement. In such a case, the user cannot be certain which focusing point the camera has chosen to meter for exposure. However, in many cases, it's possible to take a preliminary test shot and review the histogram on the LCD screen to determine if there is any need for exposure compensation.

Speaking about AI Servo AF with Automatic Focusing Point Selection: In this case, the user is never certain about which focusing point is active because the focusing points do not light up while the camera is focusing. As in the case of One-Shot AF, it's sometimes possible to take a test shot and review the histogram on the LCD screen to determine if there is any need for exposure compensation.

Evaluative metering is at its best when photographing people, but it also works very well with a wide range of non-human subject matter. It may need some help from time to time in terms of exposure compensation, especially in situations where the primary metering segment is centered on subject matter that is particularly bright or dark.

I would also say that there are some situations where it might be best to take an exposure reading ahead of time and set the camera for manual exposure. This can help improve the consistency of exposure in burst mode shooting, compared to using an AE mode like Program, Shutter Priority or Aperture Priority.
__________________
Chuck Westfall
Technical Advisor/Professional Products Mktg. Div.
Consumer Imaging Group/Canon U.S.A., Inc.
***********

Best regards,

Doug

[Bart_van_der_Wolf]

Originally Posted by Doug_Kerr

As a result of my inquiry in another forum about exactly how Evaluative metering works with AI Servo AF on Canon EOS dSLRs , I received a wonderful, thorough discussion of the issues from Chuck Westfall of Canon USA. Since my original inquiry was also posted here, I posted in that thread a link to Chuck's response, and made a few comments.

Hi Doug,

Thanks for bringing Chuck's comments to our attention.

Bart

[Will_Perlis]

Doug,

I would imagine (and I have a vivid imagination) that "given emphasis" means "let's make this about a half-stop brighter than middle gray" (whatever Canon has decided is middle gray). I am imagining this because Chuck said "Evaluative metering is at its best when photographing people...".

It wouldn't be all that hard to do some experiments if one really wanted to know. I don't have the patience to do them but I'll be happy to read your results.

Will

[Peter Galbavy]

Does this apply then to the even more densely packed focusing points of the 1 series ? If so, I see a CF feature request coming on: "Highlight only the primary focus point"

Another followup, does this in any way apply to E-TTL II Evaluative flash ?

So, this might well explain the consitency problems I get with auto focus point selection (when I am lazy) and flash exposure...

[Doug Kerr]

Hi, Peter,

Originally Posted by Peter Galbavy

Does this apply then to the even more densely packed focusing points of the 1 series ?

I expect so.

If so, I see a CF feature request coming on: "Highlight only the primary focus point"

Yes, that would be very nice.

Another followup, does this in any way apply to E-TTL II Evaluative flash ?

Unlike ETTL, ETTL II does not do anything affected by the choice of AF points. (And I think there is a message in that!)

Best regards,

Doug