Understanding metering, part two: what to use, when | Ming Thein
With that background out of the way, let’s look at how the various metering options work, and what typical situations they might best be deployed under. Cameras typically have three options, or some variation upon that. Within these options, it’s also usually possible to fine tune various aspects of the meter’s operation. I’m going to leave out handheld meter operation since this is something that’s almost never encountered today. An important point to note is that all meters can be fooled by situations of uniform luminance, so don’t trust the readout blindly. Remember, meters function by averaging the entire evaluated area out to middle gray; this means if your evaluated area is meant to be black or white, you’re going to need to add or subtract some exposure compensation. For predominantly light/ white scenes, you need to add; for dark scenes, subtract. This holds true for every one of the different metering methods detailed below.
The simplest form of metering evaluates the frame as a whole, and tries to expose it to middle gray – under the assumption that there will be shadows and highlights, but these will average out. Seldom used today because you will almost always require exposure compensation (making it unsuitable for the point and shoot crowd which constitutes most of the global camera market), but has the one enormous advantage of behaving predictably under every situation.
The simplest form of meter is the spot meter. This evaluates luminosity at the desired point only, ignoring everything else in the frame. There are two important things to be aware of with a spot meter: the location and size of the spot. The metering spot’s location is either in the center of the frame, or tied to the selected or active autofocus point; the logic there is that you would typically want to ensure your subject is both in focus and properly exposed. Variations on the spot meter include types that are biased for highlights or shadows – i.e. you meter a shadow or highlight and it doesn’t turn out over or underexposed. Don’t forget to add appropriate exposure compensation
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