Keep in mind also that cameras' internal exposure meter has its own issues.
It's similar to FILM.... you can buy for example a Velvia film, rated ISO 64, though in cameras such as NIkon, you'd use ISO 50, which is about 1/3 less than what the label says.
Similarly for digital, you can't simply do spot metering using a spot meter (not from within the camera) and use the exposure reading, because they'll vary between the camera brands. You have to compensate by a factor from the reading.
Rules such as "metering off snow and add +2 stop" is fine, however, for digital, if you use the in-camera metering system, you may have to adjust it up by 1/2 stop or lower by 1/2 stop.
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