As I said in the previous tip, the matrix or evaluative meters that are in virtually all cameras these days are very accurate even with scenes that would have many seasoned photographers scratching their heads. There are some situations though when metering only a specific region of the frame will provide more reliable results. One of the options for those situations is center-weighted metering.
As their name implies, these meters concentrate their readings on a small central region in the viewfinder. That are is typically about 8-10cm across, though on some cameras (like my Nikon D90) you can select from three different sized areas--8mm (the default), as well as 6mm and 10mm. The meter doesn't meter exclusively from that area, but typically gives that area about 75-percent of the "weight" or importance and about 25-percent to the remainder of the frame.
The advantage of center-weighted reading is that you can tell the camera to give more importance to one specific subject or area within the frame. In this shot of an old New England headstone, for example, I knew that the slate stone would be a perfect middle tone (middle tones provide the most accurate readings with any meter) and that the snow might otherwise fool the meter into thinking there was more light than there really was and underexpose the stone. The meter read the entire scene, but put most of its emphasis on the middle-toned subject. The meter reading was perfect. The matrix meter might have provided just as good a reading, but why risk it if there is a more accurate method?
Center-weighted metering works best when there is a mid-toned subject of small, but not tiny, size within the frame. By reading off that subject and then using your meter-lock function (usually just holding the shutter-release button down halfway), you can lock that reading and get an accurate exposure for the entire scene.
In the next tip we'll talk about taking meter readings when the subject you want to read is especially small.
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