Exposure locks when pressing shutter release halfway in evaluative metering mode

Started Jan 5, 2009 | Discussions thread
Lemming51
Lemming51 Forum Pro • Posts: 14,596
Re: why exposure locks with half-press in evaluative metering mode

Agree with Pete4's explanation. Evaluative tries to determine the subject and its proper exposure via the active focus point and the other metering segments. Recomposing does not change what will be the proper exposure of that original subject.

Pete4 wrote:

... However, I'm little surprised how strong influence
the focusing point has on overall exposure, after all I would expect
strong correlation from center weighted and spot metering modes, but
evaluative mode would suggest evaluation of the whole scene and
average the exposure for whole picture (at least I thought that what
it supposed to mean) and therefore focus point shouldn't affect the
exposure by that much, unless you're introducing large area of the
sky or similar, during recompose?

Evaluative is not a simple averaging routine like CW. In measuring and comparing the 35 segments and the focus point it tries to make a determination of whether the subject is back or front lit, dark or light toned, etc. and applies its own exposure judgement rather than just averaging to 18% gray. When you re-compose and re-meter, the active focus point is on a new subject and a new determination is made as to what it might be and how to expose it.

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