Taking accurate light readings is very important in getting good exposures, whether you're metering in a manual mode or your automatic meter is doing the work for you. Metering scenes of average brightness, where nothing is radically darker or brighter than the majority of tones in the scene, is fairly simple and most built-in meters do a very good job.
Problems arise when there is excessive contrast in a scene, because the dynamic range (the dark-to-light contrast range) extends beyond the camera's ability to record those tones. One type of contrast situation that all cameras have a tough time with is what's called a specular highlight. These are extremely bright spots in a scene, created by a direct reflection of the sun or some other light source (like flash reflecting in a mirror), and they cause the meter to grossly underexpose (i.e. give too little exposure) an image because they make the meter think that the entire scene is brighter than it is.
There's really no safe way to meter with a specular highlight other than to exclude it from your composition during the metering phase. In the shot here, for example, I wanted the reflection of the sun in the water, but I knew that if I metered with that spot in the frame, the whole scene would have been drastically underexposed. Instead, I took a reading from the boat and the water (which already constituted a fairly contrasty scene) while carefully excluding the bright spot in the water. Then I simply locked in that reading by holding the shutter-release button halfway down and recomposed the scene.
Beware any very bright pockets, especially around reflective surfaces, and you'll get much more consistent exposure results.
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