5DII metering mode and exposure lock question

Started Jun 24, 2009 | Discussions thread
Lemming51
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Re: why AEL is decoupled from focus lock
In reply to Tessarboy, Feb 1, 2013

Tessarboy wrote:

In a walk-around situation however, I prefer the said simultaneous lock with 1/2 press of the shutter. It allows me to react faster with some unexpected action ...

There still is a misunderstanding. 'Restriction' was not used in that sense, but in the technical sense that Canon only offers simultaneous AF + AE lock with 1/2 Press shutter button in combination with the single Evaluative Metering Mode.

The question remains : Why no simultaneous lock with Spot Metering, Average Metering .. ?

No simultaneous lock because your focused subject is not necessarily mid-tone reflectance. Spot, Partial, and even centerweighted only give the correct exposure if the metered area is (or averages) mid-tone reflectance (e.g. 18% gray). AEL with focus lock in these modes guarantees incorrect exposure too often for it to be the default.

Prior to EOS half-press of the shutter button locked exposure, and focus was a separate unrelated function. With autofocus, metering and focus were combined on the shutter button, so EOS (and other makers) added the separate AEL button so users could still focus on one subject and meter off another, or focus/recompose without changing the exposure.

Coincident with autofocus was the development of multi-segment "intelligent" metering systems. Canon's Evaluative, Nikon's Matrix, Minolta's Honeycomb, etc. do not simply average to 18% gray. The segments are analyzed to make a determination as to the type of subject (at the active focus point) and calculate an exposure for it. That's why exposure lock is applied to focus lock with these metering systems.

Can someone confirm that the 5D III allows this ?

I'll defer to someone who owns one, but it certainly seems from the manual that use can choose to assign both AEL and AF to the shutter button.

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