Falcon said:
Sgt pepper, how to know which area is midtone?
Well, tonality refers to the warmth or coolness of a color. I don’t know how to explain it in my own words, but middle tone color should be neutral side of every color. E.g. medium gray, green leaf, etc.. (not-so-light and not-so-dark colors)
It's all up to you, how you want your image to appear. Work on 'stops' and know how to identify the tonality.
If you want the mid-tone to appear as is, then meter on the midtone, locked, recompose, and shoot (this is, of course, provided that the same amount of light is falling on the area you metered and your subj).
If you want the mid-tone to appear darker, then compenste the exposure by stepping down by 1 stop (depends on how dark you want).. If you want it to appear as lighter than the midtone, then open up by a certain stops.
Sulhan is right about using a flash to expose the backlighted subject against a bright background. If you will meter the dark portion of the duck’s face, you may end up blowing out the highlights. In this instance, you may meter the green leaf from the background to properly expose it, then let the flash expose the duck itself.
Personally, I always try to find a midtone with in the frame and get the meter reading from there. What if there’s NO midtone? Then, select any color tone, say, dark green (which is 1 stop darker from the medium green) and get the meter reading from there. Assuming you want this dark green to appear as dark green, then you have to compensate the exposure by –1 stop, otherwise, this will appear as medium green. Remember the your camera will always make the area that you have metered a midtone.
Hope it helps.