7D : metering problem

Started Jan 13, 2011 | Discussions thread
Topaz Contributing Member • Posts: 588
Re: Found : Metering in AI Servo diregards any tracked focus point

fpix, nice work. I think this is an interesting conclusion. Since I very much like the AF-linked behavior of evaluative metering, it's good to know this does not work during AI Servo. I never realized that before.

This made me ask another question: for shooters who use back-button focus in One Shot focus mode, does evaluative metering link to the AF point? After all, the focus was obtained before the shutter button was pressed and there is no focus lock, per se, at the moment the shutter is pressed. I wondered: perhaps back-button focus with One Shot also behaves like manual focus and AI Servo (i.e., no AF point linking to evaluative metering).

So I tested this. My conclusion: with back button focus in One Shot mode, the evaluative metering is, indeed, still linked to the AF point. First of all, it achieves identical exposure to the case where I focus with the shutter button. Also, even after locking focus, if you simply select different AF points while the viewfinder is still live, you can see the metering change accordingly.

So I agree with your 3 points below. And I would add that for point #1, it's true regardless of which button you use to start AF and regardless of how much time elapses between AF lock acquisition and shutter activation.

I also agree that this is important info for those who use the technique of tapping AF-ON in AI Servo mode to "simulate" a one-shot focus. They should realize they are losing the benefit of AF-point-linked evaluative metering.

fpix wrote:

So, last night I found the reason.

1) In Ev. Metering / One Shot the camera takes in account the placement of subject in the frame (it knows in which metering zone the focus is aquired) and the tonality of the subject itself and produces an exposure accordingly. A pretty good exposure. This can be observed if you have a light toned object and a dark one in the same scene. By focusing alternatively on the 2 objects the camera will respond with different exposures. This is ok.

2) In same Ev. Metering and One Shot when using manual focus (switch on lens) the camera produces an exposure for the whole scene, without taking in account any particular focus point. This is why there will be a longer exposure if you have a scene with dominant dark tonalities and a shorter one if you have a scene with dominant bright tonalities. Normal.

3) Now in Ev. Metering / AI Servo AF, with some AF point having focus aquired things happen as in 2). That means the camera calculates an exposure as if there would be no subject in the scene. This can be observed in a test like my 2 sample pictures attached yesterday. Thus with the first sample picture if I switch the lens to manual focus I get an exposure exactly as in sample 2 (I get a darker image). And the way around: being in situation of sample 2 if I instantly switch to One Shot I get a brighter image, exactly as in sample 1.

Now my conclusion:

  • This is definitevly not a bug. Since the manual does not state how exposure is calculated in One Shot vs. AI Servo. And for various other reasons.

  • This can be ok in sitaution like fix framing and tracking subject. It would make no sense to change exposure in serial shots while tracking the subject in the SAME scene.

  • But one should be aware tiny subjects on contrasting plain backgrounds might come out overexposed or underexposed. This is quite known anyhow. Alternatives: CW, partial or spot metering, eventual with adequate compensation depending on subject tonality.

  • Quite annoying for people (including myself) who keep the camera always in AI Servo and focus with separate AF-ON. AF-ON pressed --> AI Servo. AF-ON depressed --> One Shot. But in second case you get a different exposure. For static subject this is worse than using standard One Shot mode. So better switch to One Shot. C.Fn IV-1 allows you to aasign the DoF button for instantly switching One Shot AI Servo.

Maybe quite complicated for the normal photographer, but good to know for people trying to constantly reach best possible exposure.

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