evaluative metering question?

[down_shift13]

i've read through this forum that in evaluative metering, the focus point plays a role in the exposure. does this mean that if the focus point is on something dark i would dial in a -1 to abt -2 stops depending on the subject. same as if the focus point is on something white then would i need to dial up +1 to abt +2 to compensate?

[pierrot]

Not really (it's not spot metering). But yes it is more "spotted" and less "averaging" than the other modes. Try it by yourself. ;)

[scottbergerphoto]

The problem with evaluative metering is that while it biases the active AF point, it uses proprietary algorithyms to combine that info with the rest of the scene. So, for the most part, you just have to take the shot and adjust with the histogram. That being said, there are situations where you can pretty much know in advance how eval metering will be fooled. If you have alot of sky in your picture, you need to add exposure as the meter will think there is alot of light and underexpose the subject; and if there is alot of dark background, like a bunch of trees in shadow, the meter will overexpose your subject, so you need to decrease the exposure.
Scott

[down_shift13]

so i guess for most consistent results just use centre weight or partial coz i really dont have any idea how the camera meters the scene. anyway thanks for all the info.:)

[RDKirk]

i've read through this forum that in evaluative metering, the focus point plays a role in the exposure. does this mean that if the focus point is on something dark i would dial in a -1 to abt -2 stops depending on the subject. same as if the focus point is on something white then would i need to dial up +1 to abt +2 to compensate?

It's hard to get details these days about how evaluative metering works, but when it was first introduced, the photo mags gave the whole story. I still have my July 87 issue of "Modern Photography," that described and tested it when it first came out on the Canon EOS 650.

Basically, evaluative metering (and Nikon's matrix metering) attempts to match the current viewfinder scene with tens of thousands of scenes in the camera database.

First, it has to identify the subject. It presumes the focus point is the subject. Then it identifies the light levels in each of its measuring segments and converts that data into a mathmatic expression using a proprietary algorithm.

The camera database is made up of thousands of properly exposed photographs (Nikon claims over 90,000 in its database) that have also been converted to mathmatic expressions by the same algorithm. That's how the camera can search for a match within microseconds. When it finds a match, it adjusts for the base difference and sets the exposure.

It actually works very well, generally being fooled only in situations where the photographer actually wants an "incorrect" exposure.

But this is what you have to know: The camera will avoid being fooled by all kinds of light conditions and patterns, but just like every other meter, it will attempt to make the subject a medium gray tone. In other words, the camera says, "Aha! The subject is in front of a dark background under a spotlight with some lights in the background shining directly into the lens...and the subject is medium gray."

If you are shooting a bride in white with the groom and groomsmen in black, the camera won't be fooled by all that black, but it will still try to make the bride's dress medium gray.

So just like with any other meter, if the subject is NOT medium gray, you have to dial in some compensation, then let the evaluative system go for it.

Also, you may have noted that an automatic meter reading is locked in evaluative mode if you focus and recompose. That's because, as I said, evaluative mode has used the focus point to determine exposure...so it's not going to change from that unless you refocus.