Evaluative Meter = TERRIBLE for 'Focus & Recompose'

Started Apr 25, 2012 | Discussions thread
Kabe Luna
Kabe Luna Veteran Member • Posts: 9,493
AE-L...

...solves your problem. Automatic focus point selection does as well–if the camera finds/covers your subject with your chosen composition.

cs hauser wrote:

Focus & Recompose is a common technique among photographers. Unfortunately, evaluative metering FAILS whenever that method is employed. It doesn't matter whether the camera is using default settings, or if it's configured to separate exposure lock from focus lock (via the AF-On button).

In the following cases, we assume One-Shot AF Mode with manual AF point selection.

CASE 1: Camera defaults to locking focus & exposure with half-press of shutter button

  • We half-press the shutter button to lock both focus & exposure. We recompose. We press the AE-Lock button to re-meter for the recomposed scene.

  • PROBLEM: The new exposure will be biased heavily towards the selected AF point, as if it's still pointing at our subject. But we have already recomposed, and the selected AF point is no longer pointing at the subject! The meter will be skewed to expose for whatever random object happens to be under the selected AF point.

CASE 2: Camera is set up so that AF-On button locks focus, and a half-pressed shutter button takes a meter reading.

  • We press the AF-On button to lock focus. We release AF-On button to stop focus. We recompose. We half-press shutter button to meter the recomposed scene.

  • PROBLEM: The new exposure will be biased heavily towards the selected AF point, as if it's still pointing at our subject. But we have already recomposed, and the selected AF point is no longer pointing at the subject! The meter will be skewed to expose for whatever random object happens to be under the selected AF point.

It seems to me that evaluative metering will never work properly with "focus & recompose". I don't see any way around it. There is no custom function to unlink the selected AF point from the exposure meter. This is a particularly nasty flaw for the 5D & 5D2, since "focus & recompose" is a necessity with these cameras.

The most convenient workaround is to lock exposure before recomposing. But this will result in an exposure that is not optimal for the recomposed scene.

What the camera SHOULD be doing is memorize the meter reading at the active AF point when focus is achieved (i.e. the subject). Then every subsequent meter reading should be skewed towards this memorized value. This memorized value will not be erased until the next time the camera starts auto-focus, presumably with a new subject.

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