Since the advent of the “18% grey” principle (in the 1960’s!), reflectance meters have been incorporated into cameras. A depiction of the most popular implementation appears in Figure 6-34a (left), where the camera would weigh the reflected light coming from the center of the viewfinder more than it would the edges. This “Center-weighted” metering pattern, a Nikon invention, makes the reasonable assumption that your subject is more likely to be in the center than the edges. (Clearly, nobody at Nikon had ever heard of the rule of thirds! :-) )
Starting in the 1980’s, there were many attempts to improve upon the accuracy of the built-in exposure meter for scenes that are not “average”, such as subjects that are backlit. Usually these entailed dividing the picture into smaller areas and analyzing each of these areas in terms of absolute light measurement (with an emphasis on what’s behind the focus point) and comparing the values against each other and against a tiny in-camera database of “standard compositions” with rules like “When the metering pattern looks like this pattern, overexpose by ½ a stop; and when the pattern looks like this other pattern, then underexpose by 0.7 stops.”
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Figure 6-34: The older Center-weighted metering pattern (left) and the modern Multi-Segment metering pattern (right). (Well, truth be told the modern cameras use every single pixel as a metering area, not the 40 honeycomb pattern pictured on the right. But you get the idea.) With the advent of full-time live view, it’s hard to find a reason to use center-weighted anymore. |
This technique of dividing the frame and evaluating the exposure is commonly known as “Matrix Metering” (Nikon terminology – they invented it) or “Evaluative Metering” (Canon terminology), or “Multi-Segment Metering” (Sony) (Figure 6-34b). It tends to succeed in getting the right exposure under a wider set of circumstances than the standard, center-weighted 18% grey method. The earliest versions (Nikon) divided up the viewfinder into 5 segments; earlier Sony cameras had 40 segments, and your camera (because the metering is done via Live View) uses every single pixel as a metering segment. (Overkill, yes, but in this case it’s the easiest thing for them to implement.)
Old film professionals disliked Multi-Segment metering at first, because they had invested a lot of time understanding their 18% exposure meters and knowing intuitively when they would make bad recommendations and when (and how much) to override them. In their minds, the problem with Multi-Segment metering (and this was quite relevant in the days of shooting slides) is that you can’t possibly know how much to set your exposure compensation to since you don’t know how the camera’s meter is choosing to handle a difficult, non-average composition. (“Should I overexpose this picture of a bride in her white dress, or did the Evaluative metering already take that into account?”) And so, for these folks, camera manufacturers left the old Center-weighted metering system in the camera as a selectable option. It was consistent and predictable and it would never try to second-guess you.
With the advent of digital, and especially full-time live view, you’re not working blind anymore, plus the Multi-Segment metering algorithms have a great track record of making the right choices in a wider set of circumstances. That’s why I keep my camera set to Multi-Segment metering all the time. And when the light is really harsh and non-average, like a predominantly dark scene, I temporarily switch to either Spot metering (explained in the next section) or manual exposure. I never use center-weighted.
