Tuesday, September 9, 2008

Understanding Metering modes


In our series of articles about "Mastering the basics" we learned about the different settings to use to get the right exposure, but we never talked about the way we metered available light.

Now, we're going to take a look at the camera's metering modes.

Every modern camera has a sensor that measures how much light is getting through the lens. In most cameras we can chose how we want the camera to do it. There are 3 different metering modes:

Matrix metering: The most used of all modes, the camera measures light from all points in the scene, and attemps to set the exposure in a way that the whole scene is properly lit.
This is a very easy to use and effective metering mode because the largest possible part of the image will be properly exposed. Similarily to the auto mode, though this metering mode has its limitations when we want to have some advanced control over the resulting image.

Center weighted spot metering: This mode comes in very handy when shooting differently lit scenes. Imagine for example, that you're shooting at a concert. The whole scene is dark, except for the performer in who is lit by spotlights. Using the matrix mode, the camera will try to expose the biggest part of the scene, as a result the preeminent darker zones will be properly exposed, while the performer will be overexposed. This will be in many cases far from the ideal result. In these situations we'll use the center weighted metering mode, now the camera will prioritize the area in the middle and expose properly for a portion of approximately 25% of the image around its center.


Photo by sint

Central spot metering: This is the most selective mode, the camera will expose properly for a point situated in the exact center of the image. The rest of the scene will not be taken into consideration. I do not recommend this mode in most of the cases because light can change a lot from one point to another and the readings won't be very useful if we use one point only. It can be a good mode to use if we want to select an exact point for the camera to expose properly though which can be useful for example for creating silhouettes or for complicated light situations.



Now, when you read about the last two metering modes you were probably asking yourself, what about the framing? What if I want the performer in the concert to be properly exposed, without framing him in the middle?

Well, camera designers were aware of this problem and that's what the Auto Exposure Lock button is for. Most DSLRs have this button that allows you to lock the exposure settings once your camera has taken the light readings and adjusted its settings accordingly. This will allow you to take the light readings from the performer by placing him in the center and then, with the AE-Lock button pushed frame the shot as you want to.

Summing up: Center weightered and center spot metering modes, allow you to chose the part of the image you want to be correctly exposed, while matrix metering mode will try to have the whole image exposed properly.

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