10D Active Focus Point (without flash) influence exposure?

[dpanicc1]

Without flash, is exposure biased by the active focus point? I know it is with flash, but not sure it is without. Seems tough to test if biased slightly.

Thank you,
Dan

[drisley]

AFAIK it is.

[dpanicc1]

What? Am I missing something or was that a stupid question?

[drisley]

No, that is why I said "AFAIK it is."
Atleast that is how it works on the 300D

[dpanicc1]

Ah, I got you now. In the center weighted mode I don't see anything about biasing linked AF point(s) to the subject (that's necessarily the case, indirectly, if the center focus point is active; and I know all the focus points are linked together and tied to the 35 metering zones of the 10D (according to the manual). But I've never read anything that I could recall where the active focus point locked onto a subject biases exposure. However, the active focus point DOES bias the flash output and that's a seperate issue.

Thanks,
Dan

[scottbergerphoto]

Without flash, is exposure biased by the active focus point? I know it is with flash, but not sure it is without. Seems tough to test if biased slightly.

Thank you,
Dan
Dan,
You are talking about Focus Point Linked Spot Metering. That is a feature usually found only in high end cameras (F5, 1D, 1D Mark II). The 10D doesn't have spot metering let alone focus point linked spot metering. (I love my 10D, but it is not a Series 1 camera).
Usually spot metering when present is limited to the small central circle in the viewfinder. With Focus Point Linked Spot Metering, the metering is based on a reading at a single manually selected AF point. On the 1 D Mark II, to get focus point linked spot metering, you need to first select spot metering and then choose a manually selected AF point.

As a side point, with the new ETTLII on the 1D Mark II, the flash metering is totally independent of the focus points.
Regards,
Scott

[drisley]

Hmmm, actually, in center-weighted mode the AF points dont actually affect exposure (hence the name).
The AF points will only affect the exposure in evaluative, and partial metering.

[scottbergerphoto]

Hmmm, actually, in center-weighted mode the AF points dont actually affect exposure (hence the name).
The AF points will only affect the exposure in evaluative, and partial metering.
Sorry, but that is incorrect. The metering of ambient light on the 10D and 300D have no direct relation to the active AF points in any exposure modes. The metering in the various exposure modes are related to their corresponding circle in the viewfinder. The only exception is that in Evaluative metering, Manual focus, the exposure is biased to the Center AF point whether or not that is the active AF point.
Perhaps you are confusing the 35 metering segments with AF points.
Scott

[dpanicc1]

Scott,

I agree with you. Based on what I've read and understand you're correct. ACTIVE focus points don't bias exposure--even in Evaluative Metering: they're all averaged "somehow" into the 35 metering zones using Canon's magic algorithm. Exception would be when manual focus and evaluative metering are used together, then exposure is based on the center AF point.

Flash is a different story: the flash exposure system IS BIASED by the active focus point.

Thanks,
Dan

[drisley]

Well, the active focus point does affect metering in evaluative mode, atleast on the 300D.

Just now I tried a test.

I have a white wall with a mirror that has a thick black frame.
I pointed the camera so that I have half of the frame in the picture.
With the active AF point over the white wall, I get an exposure of 1/10s.
When I change the AF point (but dont move the camera), so that the AF point is now over the black frame, I get an exposure of 1/6s. When I change the AF point back to the center, so that it's over the white wall, I once again get an exposure of 1/10s.
This is in AV mode with evalutative metering.
This sure sounds like the AF point affects the evaluative metering to me.

In addition, here is a quote from Steve's digicams (which is no doubt a quote from Canon)...

"Canon's exclusive 35-zone Evaluative metering links to the active focusing point and is the default metering pattern in AE modes, while center-weighted average metering is automatically selected in Manual mode. Central-area partial metering (measuring 9.5% of the picture area) is available on demand in Program AE, Shutter-priority AE, Aperture-priority AE, Auto Depth-of-Field AE and Manual"

[drisley]

Ok here is a test I tried.
Both are shot with Evaluative Metering in AV mode.
Exact same picture, the only diff is the AF Point.
Why are the exposures different if the AF Point does not affect exposure?
Or is this not at all what you are talking about?

http://www.mts.net/~lftbrain/center.jpg

http://www.mts.net/~lftbrain/right.jpg

[petiot]

hummmm i think some will have to review their manual after drisley's demonstration!!

come on, do you home work!! ;)

[drisley]

You know, I get the EXACT same exposure in Center Weighted Average (M) mode as I do in Evaluative Mode (AV) with the center focus point selected. There is no diff. Almost makes Evaluative Metering sound like a scam. I used to get different readings with my G3 in CW and Eval modes, but on the G3 the metering was definately not linked to the AF points as with the 10D and 300D.

Oh, I just discovered the same statement about the 10D as with the 300D statement above ....

Canon EOS 10D....As with the D60, three metering modes are provided: 35-zone evaluative linked to the AF points (evaluative metering is based on the center AF point when you focus manually), center-weighted and 9% partial, but improved algorithms make the excellent evaluative system even more accurate.

[dpanicc1]

Drisley,

Thanks for running the test. It is confusing. The manual talks about it on page 75 but it's not clear--to me anyway. Hard to know what Canon's 10D evaluative enbedded code is doing to exposure--they certainly do not make it clear. I'll stick with center-weighted in M and Av and I'll decide the tone of the photograph using compensation. Not a good practice to compensate evaluative--too unpredictable.

Regards,
Dan

[scottbergerphoto]

This document about the Canon Evaluative Metering Algorithm is instructive. http://www.camera.canon.com.my/archive/photography/art/art14/2.htm
"Basically, the EOS 300's 35-zone Evaluative metering system takes into consideration which of its 7 AF points have locked onto the main subject as well as the differences in brightness between the main subject and the background. If more AF points are concentrated on a darker subject than a brighter one, the camera will assume the former as the subject and calculates the settings to give optimum exposure for that picture.

Simultaneously, if you concentrate more of the camera's AF points on a brighter subject, the camera will give preferences over it and less on the darker elements. This is fine if you have concentrated the AF points on the correct subject, otherwise you will end up with overexposed or underexposed pictures. Photo H is an example of how the EOS 300 gave a correct exposure on the mannequin biker despite the brighter elements in the background."
So apparantly, the active AF point(s), do bias exposure as part of a complex algorithm which also takes into account backround lighting.
Scott

[drisley]

Hasn't history taught you anything?
Never question what I say as I am always right! :lol: :lol: :lol:
(actually it's nice to be right for ONCE) :lol: