Does focus point change metering?

[tonyw3026]

I seem to have read that using just the center focus point instead of 9 point, changes the way evaluative metering works. I have a 300D and a 20D and the manuals are not clear on this.
Can anyone help? Are the 300D and 20D different?
Thanks

[slin100]

I can't provide answers for the 300D or 20D, but on my 10D evaluative metering is biased by the active focus point(s). If you switch the lens to manual focus, the camera appears to either switch to biasing the center focus point or center-weighted averaging--I can't tell which.

Learning this has made me consider switching metering modes to center-weighted averaging.

[tonyw3026]

Yes, my 20D manual says that for evaluative metering "After detecting the main subject's POSITION, brightness, etc ........... the camera sets the proper exposure"
This implies that the metering is biased by the focus point(s) being used. If this is correct, I wonder what is the nature of this bias? I also wonder if the bias is enought to effect the choice of 9 point versus center point focusing?
The 300D manual makes a similar statement so the two seem to be the same.

[slin100]

I'm not sure anybody knows the extent of the bias. But whatever it is, it'll never be enough for me to opt for the uncertainty of wide-area, multi AF-point focusing, especially when DOF is narrow. A 1-series camera, however, I might consider since the focus points are closely spaced.

[RDKirk]

I seem to have read that using just the center focus point instead of 9 point, changes the way evaluative metering works. I have a 300D and a 20D and the manuals are not clear on this.
Can anyone help? Are the 300D and 20D different?
Thanks

Back when evalutative and matrix metering first came out, the photo magazines described how they worked in some detail. Unfortunately, you'd have to go to the library and dig through the microfiche collection to find those descriptions...but I do still have my old July 87 issue of Modern Photography that had a stripdown report of the then-new Canon 650.

Evaluative (or matrix) metering uses the focus point to identify the subject, then it evaluates light levels in different segments of the image (35 segments for the 20D) and uses a proprietary algorithm to convert the light pattern to a mathmatic equation.

The cameras have a database containing the equations of tens of thousands of photographs (Nikon claims 90,000 photographs) that have been converted using the same algorithm. Essentially, the camera matches the current scene with a similar photograph in its database to determine exposure.

The camera presumes the point you focused on is the subject, and like all meters, it will attempt to reproduce that subject as medium gray.

Thus, if you photograph a bride in white surrounded by groomsmen in black with brillian sunlight backlighting them, the camera will recognize that the bride in white is the subject, and it will recognize that there is heavy backlighting, but it will still reproduce the white gown as medium gray, like all meters.