Your first version is inaccurate. Your second version is getting warmer, but it's still not quite there. Perhaps a better way of expressing the issue is as follows:
1. Evaluative metering reads all segments of the metering sensor.
2. Primary emphasis is placed on the metering segment overlaying the active focusing point, or the center focusing point when the camera is set for manual focusing.
3. Secondary emphasis is placed on the metering segments directly adjacent to the primary metering segment.
4. Tertiary emphasis is placed on all other metering segments.
Evaluative metering uses a variety of algorithms to determine if there is a need to apply exposure compensation to the reading obtained at the primary segment. The algorithms are proprietary and confidential for competitive reasons, but we know that they look at various factors including overall light level, backlighting, spotlighting, and an estimate of relative subject size, as revealed by a comparison of brightness values for each metering segment. They may look at other available data as well, but if so we're not sure exactly what that data may be or how it is used. We know that there are limits on the maximum possible range and gradation of exposure compensation steps that evaluative metering can apply, and we know that the specifications for these factors vary somewhat according to the camera model in question, but we do not know the exact specifications for either.
That's just about as much technical info as I can provide on this topic.
That's just about as much technical info as I can provide on this topic.
That is very helpful. Thanks so much for taking the time to give such a thorough and explicit description.
To me, this is the key:
Quote:
Evaluative metering uses a variety of algorithms to determine if there is a need to apply exposure compensation to the reading obtained at the primary segment
I don't think I have ever seen that stated so clearly. That makes a lot of sense.
This is another key:
Quote:
. . . and an estimate of relative subject size, as revealed by a comparison of brightness values for each metering segment.
As you can probably tell from my earlier comments in this thread, for many years I believed that the "estimate of subject size" was based on the "cluster" of AF points that "joined" the Active one. I now realize that the only significance of the "joining" AF points is that they have a consistent result with the Active one regarding subject distance.
Best regards,
Doug
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
I just want to pass along a note of appreciation to Chuck for helping all of us Canon users better understand our equipment. So thank you Chuck for taking the time to provide detailed explanations.
Just another thanks to Chuck for all his knowledge he shares with us. After many years of shooting with the "Rebel" series I'm moving up to the 50D that should arrive this afternoon.
Thanks!
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
I've just bumped into this thread and found this information on evaluative metering quite fascinating - and a revelation.
Thank you Chuck Westfall for the info and well done Doug for the putting up the question (I used to enjoy the indepth discussions on DPreview regarding the inner workings of the 300D all those years back).
White Balance so easy, even our 5 year old can do it.- Melissa Strickland
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