If you wish, you can skip this explanation.
However, if you read on, you'll be one of the few photographers who understand completely how their light meters work.
Measuring Reflected Light Causes Errors
Light meters usually measure light that's reflecting off of the subject.
Light meters get confused because the subject absorbs light—sometimes a lot—and sometimes very little.
Example - White bowl & Black Velvet
In this example, there are seven stops of light falling on the scene.
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Seven Stops of Light Falling on the Scene 16 26 36 46 56 66 76 |
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Not all of those seven stops are reflecting back to the camera, however.
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132333435363 |
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132 |
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Pity the poor light meter.
How can it decide what to do when there's such a big difference—six stops from the white bowl—and two stops from the black velvet?
If You Use the Bowl to Set the Exposure
If you use an exposure setting made from the light coming from the bowl, this is what you get:

The light meter "sees" that there's lots of light reflecting off of the bowl.
The light meter "says:"
Wow, it's bright out there. I better turn down the exposure.
If You Use the Velvet to Set the Exposure
If you use an exposure setting based on the light from the velvet, this is what you get:

The light meter "sees" that there's very little light reflecting off of the velvet.
The light meter "says:"
Wow, it's dark out there. I better turn up the exposure.
How do you get the correct exposure?
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