7D : metering problem

Started Jan 13, 2011 | Discussions thread
OP fpix Regular Member • Posts: 346
Found : Metering in AI Servo diregards any tracked focus point

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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