Beecher's Handouts /
Light Meters Are Stupid / 15.3 - Why Are They Stupid?
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.
|
Seven Stops of Light
Falling on the Scene
16
26
36
46
56
66
76
|
 |
Not all of those seven stops are reflecting back to the camera,
however.
|
Six stops of light are reflected back to the
camera light meter from the bowl.
|
|
|
One stop of light is absorbed by the bowl.
|
|
|
132333435363
|
|
|
Two stops of light are reflected back to the
camera light meter from the bowl.
|
|
|
Five stops of light are absorbed by the black
velvet.
|
|
|
132
|
|
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?
