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What is Light Metering?
Light metering involves the process of determining your shooting options based on the lighting conditions of your composed photograph. You can, of course, use a separate light meter which will measure the light and give suitable f-stop and shutter speed values to shoot, which the professionals do. However, all modern digital SLRs come with an in-built light metering system that the user can make use of. This is what we call camera metering - the camera measures the amount of light and determines optimal exposure options.
Since photography is an interplay of colors, composition and light, the way you measure the light has a profound effect on your photograph. When you compose a photograph, you have a pretty good idea of what you’d like the end results to be, however, your camera may not “cooperate” in providing you with these results. This where camera metering comes in as an integral part of your photography skill set.
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Camera Metering Modes
Most digital SLRs come with the following metering modes and choosing them wisely can add or take a lot from a photograph.
1. Spot Metering
2. Center Weighted Average Metering
3. Partial Metering
3. Evaluative Metering or Matrix Metering
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As the name suggests, Spot Metering involves selecting a spot in the frame where the luminance values are used capturing the entire shot. At times, camera metering may get confusing due to complex lighting conditions, this is where Spot Metering comes in very handy. Using Spot Metering, you can light your picture in such a way that your main point of interest in the frame is the one for which the light is adjusted and therefore can give a beautifully lit picture in relation to your main subject. A lot of photographers also employ this technique to make great compositions where the point of interest is focused by the lighting setup.
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Center Weighted Average Metering
Like Spot Metering, it should be easy to deduce what this camera metering mode would do. Center Weighted Average Metering takes the center of the frame as reference and an average of the surrounding area to calculate the exposure. Although this has been the default choice on cameras for sometime now, it's quickly being replaced by Evaluative/Matrix Metering modes, which are considered "smarter". However, the advantage that Center Weighted Average Metering offers over the latter is predictability. Smart metering techniques, like Evaluative/Matrix Metering, make use of complex algorithms that may or may not be the ideal option for a particular condition. They work under the presumption that the exposure that the camera automatically selects is the right exposure, which may not always be the case. This is where Center Weighted Average shines, given a scene you can predict the exposure values that might come up by having a particular object in the center of the frame. This allows us to compose a shot with much greater control over the lighting conditions.
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Partial Metering is essentially another version of Spot Metering. The difference being that the region selected for metering is bigger in the case of Partial Metering. While taking pictures of subjects not uniformly lit, it is recommended to use Partial Metering. A good example would be a portrait against a heavily lit background. If not metered properly, a portrait in a heavily lit background would give a silhouette! Try the same with Partial Metering and you'll see the difference.
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Evaluative Metering/Matrix Metering
Evaluative Metering (Canon) or Matrix Metering (Nikon) is the default metering mode on the camera, and for a reason. With your camera set on this metering mode, you can shoot most scenes and expect to come out with a decent picture. The light metering of this kind picks up light values from several point of reference from the frame you've chosen and gives a more or less uniformly lit picture. This setting is suitable for most shooting conditions and should be your choice if you don't know which other metering mode to choose. In fact, in some digital cameras, like the Nikon D80 and the Canon Rebel XSi, this metering mode is absolutely right on the money, no matter how complicated a scene is lit.
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Experiment & Discover Your Digital Camera's Metering Modes
Now that you've read all about the different camera metering modes, you can now start playing around and experimenting to master your photography skills. I recommend using these metering modes especially when shooting in conditions that are lit trickily.