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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 in cameras measure light that's reflecting off of the subject.

Camera light meters can't measure how much light was absorbed by the subject.

White snow in sun absorbs very little light, and reflects a lot of light.

So, the light meters "sees" a lot of light, and makes the exposure too dark.

Black gloves absorb lots of light.

So, the light meters "sees" very little light, and makes the exposure too bright.

Example

In the example below, there are seven stops of light falling on the scene.

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Not all of those seven stops are reflecting back to the camera, however.

White Bowl

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Six stops of light are reflected back to the camera light meter from the bowl.

One stop of light is absorbed by the bowl.

Two stops of light are reflected back to the camera light meter from the bowl.

Black Velvet

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Five stops of light are absorbed by the black velvet.

Two stops of light are reflected back to the camera light meter from the black velvet.

What the Light Meter Sees

So, there are seven stops of light illuminating the scene.

But, if you take a light reading from the bowl, the light meter will see six stops of light.

Or, if you take a light reading from the black velvet, the light meter will see two stops of light.

What's a Light Meter to Do?

Pity the poor light meter.

There are six stops of light from the white bowl.

There are two stops from the black velvet.

How can it correctly decide what to do when there's such a big difference in light reflecting from the scene?

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.

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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.

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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?