Quote:
Originally Posted by Tiberius
...Light meters are designed to reproduce scenes so that they have an overall tonality of 18%....
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Actually,
12%, according to Ctein,
who says he got it from the Kodak engineers who design light meters and their specs.
Don't get too caught up in the numbers. Just know the concept of "middle gray" and the effect of the metering on autoexposure modes when it tries to set a certain value to it.
For the sake of simplification, let's say that your metering mode averages out all the values in the frame, and then sets the average value to middle gray. As long as the frame contains a good wide tonal range between black and white with relatively even distribution, this is a reasonable way of setting the exposure. But if your scene is predominantly black (like shooting at night)? Then this same mode is going to be setting a very dark value to middle gray, and the scene will be overexposed. Similarly, if you're shooting a predominantly white scene, a very light value will be set to middle gray, and you'll be underexposed.
This is why we have Manual mode and exposure compensation, and evaluative, center-weighted, and spot-metering metering modes.
