Hello John,
In the original article I tried explaining the way that the camera's metering worked when faced with a completely black or completely white surface initially to try and get the concept across. As you quite correctly point out, using spot metering off a black or white point in the image will give the same tendency towards dark grey or off-white as a completely black/white surface.
If you think about multi-segment metering, it is like a whole bunch of individual spot meters (35 in your case). Each segment will take a meter reading and the same rules will apply for each segment, the exposure will only be exactly correct if the surface is a mid-grey. However, the camera will take all 35 readings, off all the different segments and try and compromise to give a balanced exposure over the whole picture.
The same rules apply as I’ve explained in the original article, it’s just that there’s a debate going on inside the camera between 35 delegates before the camera comes up with an agreed solution! If we take the classic case of air-show photos (Stephen Fox/Snappy will jump on me if I get this wrong!), we have a relatively dark plane flying in a bright sunlit sky. Let us say that 25 segments are metering the sky and 10 are metering the plane. The camera does not know that the subject is the plane in a bright sunny sky, it just knows it’s a given amount of light that’s coming in to it. The metering is calibrated to give a correct exposure off a mid-grey surface, the sky is a lot brighter than that, so there is a tendency for the camera to underexpose the sky. “What about the plane?” you ask. In our example, 10 of the segments will be saying “Errrm, we’ve got a much darker bit in the middle here, can we have it a bit lighter on the exposure please?” The camera will bias the exposure a little towards the brighter side having had the input from the 10 segments covering the plane, but the 25 segments will win the vote against the 10 and the bias will not be enough to ensure the plane is exposed correctly.
Knowing that this compromise is taking place between your 35 segments, you should evaluate which part of the picture you wish to be most accurately exposed. Look at the light/dark tone of the part you wish to be exposed correctly. Look at the remainder of the picture and decide if it is a lot brighter/darker than the part you want to get right. Look at the relative areas of the part you want to get right versus the remainder of the picture. Knowing that your camera will try and balance the whole image towards a compromise in the middle, you should be able to adjust the compensation accordingly.
Some examples:
Small mid-tone subject against bright larger background.
Spot metering of the mid-tone subject will give the correct exposure, leaving it on multi-segment will result in the subject being slightly under exposed as the majority bright background wins the vote. Exposure compensation to apply: A small amount of positive compensation.
Small mid-tone subject against a dark larger background.
Spot metering of the mid-tone subject will give the correct exposure, leaving it on multi-segment will result in the subject being slightly over exposed as the majority dark background wins the vote. Exposure compensation to apply: A small amount of negative compensation.
Small bright subject against a dark larger background (see post #2 in the original article).
Multi-segment metering will result in the subject being over exposed as the majority dark background wins the vote. Exposure compensation to apply: A moderate amount of negative compensation.
Small dark subject against a bright larger background (see post #2 in the original article).
Multi-segment metering will result in the subject being under exposed as the majority bright background wins the vote. Exposure compensation to apply: A moderate amount of positive compensation.
Exactly how much to apply will vary depending on the exact circumstances, amount of light/dark areas and just how far off mid-grey the subject and surroundings are.
Hope this helps.
Duncan.
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