Confusing hor?
I (newbie too) try to simplified. Correct me if I'm wrong.
Assuming AVERAGE metering mode is used, when you meter on a all black surface, the result will be 18% grey. Likewise, on an all white surface, same result.
It is because the cam will average all into 18% grey.
so when your subject and background is of contrasting tone , then the cam will be fooled as it takes the 'average', resulting main subject wrongly exposed.(e.g. subject indoor beside a window with strong light from outside, will have the subject under-exposed).
So SPOT or CENTRE-WEIGHTED metering modes possibly used for such scenarios, where the cam meters on where you point at, disregard to other areas. For the above eg, the subject will be properly exposed, but scene outside windows over-exposed.
To meter on a neutral looking areas needs experience, i.e. you have to see things in black and white, and aims (spot mode) at areas that's 'suppose' to be 18% grey.
Some says the back of hands is about there (18%). I personally doubt due to the vast variations (e.g. ang-mo? indian?).
But it depends on what you want to achieve. If you explicitly want only to capture the subject's silhoulette, then spot metering off the background (for the above example). Or meter off the window and use flash, then can capture all.
Note that there are other modes of metering, named differently, and perform differently (proprietory to different cameras).
