Exposure compensation

A friend asked me the other day what the exposure compensation feature on his camera did, and why he might need such a thing. I’m not bound to any particular agenda for this blog, so I thought I would have a bash at explaining.

Some facts

  • Cameras have light meters which they use to measure how bright the light in a scene is and from this, they work out the combination of shutter speed and aperture that will allow the right amount of light to reach the sensor (or film).
  • Cameras assume that the scene you’re pointing them at is of average brightness which is the most common situation.
  • Cameras will work out their exposures to render the scene as being of average brightness.

The problem comes when the scene is brighter or darker than average. The camera has no way of knowing that you are shooting a snow scene, which is supposed to be bright, or a black cat in a coal cellar at midnight, which is supposed to be dark.

So what does this mean?

The camera is going to get the exposure wrong when faced with a scene that is not of average brightness. Remember that the camera has assumed that the scene is of average brightness, which is why you so often have snow scenes which are darker than they should be, where the snow is blue or grey, rather than white. The same thing applies to dark scenes and is why you so often get pictures where black kitty on a dark sofa becomes grey.

What to do

How to fix the problem is through the use exposure compensation, which is just a way of telling the camera to let in more or less light than it thinks it should.

1)    Take the shot at the setting the camera suggests.

2)    Check out the picture on the review screen and adjust the camera if the picture is darker or lighter than it should be.

  • Dial in positive compensation (+) to let in more light if the picture is too dark.
  • Dial in negative compensation (-) to let in less light if the picture is too light.
The picture in the middle is taken without any exposure compensation and because the camera didn’t know that the scene needed to be brighter than average, it got its sums wrong. By telling the camera I wanted it to let in more light (positive compensation), the scene on the right is brighter, which is how I prefer it. The scene on the left shows how the picture looked when I told the camera to let in less light (negative compensation), to make it darker. Click to see a larger version of the picture.

How much compensation to dial in?

When taking a picture of something which is predominantly bright, like a snow field for example, I would start by dialing in 1 stop of positive (+) compensation, and increasing that by thirds of a stop until I get a result I like.

When taking a picture of something which is predominantly dark, like a black cat for example, I would start by dialing in 1 stop of negative (-) compensation, and decreasing that by thirds of a stop until I get a result I like.

NOTE: The exact amount of adjustment you should make is pretty subjective. If the picture on the review screen looks right to you, it is right. If not, change it until it is right.

NOTE 2: Don’t forget to return the camera to Zero compensation when you’ve finished shooting a particular scene.

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