R5 Mark II + "Detect priority AE while AF"

[thegios]

Apologies, I've tried asking this before, but haven't got a clear answer yet, so I'll try asking again...

Part 1

Is it true or not that Evaluative Metering considers the whole scene but with a bias on focusing point? I am asking because this is not documented on Canon manuals but many photographers say so. This is also supported by Canon manual (thanks Canon for making things ambiguous) in the AE Lock description: in Evaluative Metering the AE Lock considers the point in focus.

Part 2

If we all agree that Evaluative Metering is biased on point in focus , then what is "Detect priority AE while AF" for? This is a new option introduced with R5Mk2 and works only under two concurrent conditions: Evaluative Metering + a SUBJECT is in focus, where subject is a person, an animal or a vehicle. SUBJECT is key here, because for "Detect priority AE while AF" a chair in focus or a house in focus or a tree in focus is not a SUBJECT.

- - -

So, given we are in Evaluative Metering, I'll try to give my interpretation of three possible scenarios, based on some tests:

1. "Detect priority AE while AF" is OFF: metering considers whole area with a bias on point in focus.

2a "Detect priority AE while AF" is ON + no SUBJECT in focus: same as 1 (metering considers whole area with a bias on point in focus.)

2b. "Detect priority AE while AF" is ON + SUBJECT in focus: metering considers ONLY THE SUBJECT, not the whole area with a bias on the SUBJECT but ONLY THE SUBJECT, that is a sort of spot metering on the SUBJECT in focus.

- - -

What do you think?

[Waddizzle]

“ 

So, given we are in Evaluative Metering, I'll try to give my interpretation of three possible scenarios, based on some tests:

1. "Detect priority AE while AF" is OFF: metering considers whole area with a bias on point in focus.

2a "Detect priority AE while AF" is ON + no SUBJECT in focus: same as 1 (metering considers whole area with a bias on point in focus.)

2b. "Detect priority AE while AF" is ON + SUBJECT in focus: metering considers ONLY THE SUBJECT, not the whole area with a bias on the SUBJECT but ONLY THE SUBJECT, that is a sort of spot metering on the SUBJECT in focus. “

You seemed to have had clarity in your previous post with a similar name and question.  Now you seem to be overthinking this and leading yourself to tangential false conclusions and assumptions.  

The behavior changes depending upon your AF Mode settings and the selected AF point(s).

1. If the AF is off, then there won’t be any points in focus. No active AF points should be visible. When no AF points are locked when the shutter fires, then the exposure biased to the Center AF point. 

2a) If the AF is active, then every visible AF point has the potential to achieve a focus lock, which may not occur until the shutter is fired.

2b) Evaluative Metering will always take the entire scene into account.  It should always bias the metering to the locked AF point when the shutter is fired.

All of the above is overly simplistic because it doesn’t take into account WHEN the AF point is locked, which impacts when metering is locked.

One Shot AF locks metering when focus is locked.  Servo AF locks metering when the shutter is fired, which does not always achieve a focus lock.

[thegios]

I'm asking again because it's all so confusing...

Especially if you consider "AE lock meter. mode after focus" and you have BBF and you always use servo...

All I know is that

1. Canon says Evaluative Metering considers whole area BUT some photographers say there's a bias on point in focus: where's the truth?

2. Canon says Evaluative Metering considers whole area BUT then also says that for AE Lock in Evaluative Metering the exposure centered on the AF point in focus is locked: isn't this a contradiction?

3.  How does "Detect priority AE while AF" relate to the above?

[Waddizzle]

Stop overthinking this.  It’s not possible to paint with such a broad brush because other settings can alter the behavior, as I mentioned above.

1) Both are correct.

2) It is only a contradiction when you ignore AF Mode and Active AF Point selection.

3) It doesn’t.

 

[thegios]

Ok then only one last question 🙂

When "Detect priority AE while AF" is on and a subject is detected and in focus, is the metering taken only on the subject, without considering the whole scene?

[p4pictures]

As said above evaluative uses the whole frame, but with a bias to the in focus AF point or subject. Say you put a person in front of a black background paper and light them with constant light, you will get a shot that biases to the person, but still the black background has some effect. If you know change the background to white paper then the exposure will shift a little since the whole scene is brighter, even though the person is still the same. You can do this on a table top setup with dolls or small animal figures to try for yourself. 

[thegios]

Yes this I understand and I agree, but then what's the purpose of "Detect priority AE while AF"?

This seems redundant if Evaluative already has a bias on the focus AF.

[p4pictures]

I am not currently able to test this out empirically, but since you have the camera then try this.

1. Set the camera to shutter priority, ISO auto and take two photos of the same subject in constant light one with a much brighter and another with a darker background. 

2. Change the setting for Detect priority AE while AF and redo the test. 

Post the exposure settings for each of the four images.

Please make sure you are using evaluative metering and one shot AF, but don't recompose the frame after the focus locks. Also if you use back button AF then you must keep the button pressed when the photo is taken or the AF will be inactive and exposure not locked. It might be worth also trying this with servo AF too.

[thegios]

With auto iso couldn't get a stable result so I put my wife (white skin but darker than the white background, black hair) against two different backgrounds (white and black) and I set:

-evaluative

-oneshot

-Tv 1/100

-ISO 4,000

I run the test several times always getting the same consistent results.

Aw) Detect priority AE while AF off + White background: f/7.1

Ab) Detect priority AE while AF off + Black background: f/2.8

> This makes sense as it proves that the whole area is indeed taken into account, because a white background gives a smaller aperture. This does not prove, though, that focus point is taken into account (or there's a bias on focus point).

Bw) Detect priority AE while AF on + White background: f/6.3

Ab) Detect priority AE while AF on + Black background: f/4.0

> I'm not sure how to interpret these slight differences: with a white background the aperture is still smaller, so I assume that the whole area is still considered, but overall the aperture with Detect priority AE while AF on are smaller with both backgrounds compared to Detect priority AE while AF off, which makes me think that metering is considering the subject.

Let's compare Aw with Bw, so Detect priority AE while AF off vs on with the same white background, with focus on a white skin face: in both cases the aperture is average, because the white background plays indeed an important role, but when I turn Detect priority AE while AF on the aperture becomes a little wider, probably because the skin is darker than the white background so a wider aperture is necessary.

My interpretation:

> Evaluative + Detect priority AE while AF off: only the whole area is considered, no bias on the point in focus.

> > Evaluative + Detect priority AE while AF on: the whole area is considered with a bias on the subject in focus.

[p4pictures]

The part…

Especially if you consider "AE lock meter. mode after focus" and you have BBF and you always use servo...

This custom function is applicable ONLY when one shot AF and evaluative metering is used. Since the dawn of the EOS system, Canon have linked one-shot AF focus complete signal with exposure lock. The idea being that with one shot AF, photographers focused on a subject then recomposed the frame, so it makes sense to bias the exposure to the AF point and lock the exposure in that case. Since around the EOS 6D DSLR the custom function was added to allow photographers to have the same automatic exposure lock with one shot AF for any of the metering patterns. Such an approach is mostly helpful if a photographer uses the central AF point and say spot metering as this only measures from the centre. Without this custom setting, they would focus, lock the focus, recompose and the metering was now pointed at a different part of the scene. 

This function is independent of BBF or shutter button being used, and completely irrelevant if you use Servo AF since there is no "locked" focus signal from servo AF.