Basic Concept: Digital Camera Metering in Depth

In a previous column, I posted about exposure and how cameras use sophisticated algorithms to choose the proper exposure. In this piece, I will cover the basics on the algorithms that cameras use. Remember that many cameras have different metering modes, but all rely on common fundamental principals and this post will focus on those. After reading this article you will emerge with a basic understanding of camera metering and some of the common pitfalls to think about.

As previously mentioned, metering is vitally important for the picture. It is the meter that decides the appropriate camera settings for every given shot. A mistake in metering will ruin and otherwise good shot. Metering algorithms were/are developed by camera manufacturers and they strive to create solutions that can meet the needs of all photographers, but even the best metering algorithms will face challenges in certain environments.

When metering a scene, the camera looks at the world in black and white. The experts who designed the systems realized that when looking at a scene in black and white, the average color they see is 17% gray. This may sound odd and so let me explain. The camera looks at the scene in black and white and then averages out all of the various shades of black and white. This average then assumes a distribution of light and dark colors that when averaged equate to 17% gray. If the scene comes back darker then the metering system will try to lighten the picture and vice versa. Why exactly this works is beyond me, but my assumption is that the engineers took many photographs and found that 17% was appropriate.

You may be saying to yourself “This is good to know, but why do I care?” I am glad you asked. The above algorithm works well in general photography; however, you need to be aware that it will not work properly in some environments and so you may need to make manual changes. Let me give a couple of examples:

You are going for a walk on a snowy day and take a picture of a tree surrounded by snow in the middle of a field. You return home to find that instead of the nice bright picture with white snow, the picture looks dreary and gray.

The problem you are running into here is that the scene has a large amount of white in it and therefore the proper exposure is brighter than 17% gray. The camera’s metering system is performing properly and tries to set the scene at 17% gray. This results in the camera under-exposing the scene and thus the resulting picture is darker than it should be. The solution to this is to use either exposure compensation or manual controls on the camera to force the camera to brighten the picture. ( Note that not all cameras have exposure compensation.) An easy way to see this for yourself is to take a picture of a white wall. It will come out gray just as the snow in the example above did. The opposite can also happen.

You are taking a picture of your black dog. You zoom in very close to get a nice view of his/her eyes. When you look at the picture, your dog’s black coat looks gray and unnatural.

This is exactly the opposite case of the snow example. Here, the scene is very black and the camera naturally wants to bring it to 17% gray and so over exposes the picture. The result is that instead of looking appropriately black, the dog looks gray. The solution here, like that above, is to use exposure compensation or manual adjustments to reduce the exposure and darken the picture.

Hopefully you found this post useful.

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