To ensure your photos are exposed correctly - not too light and not too dark - your Digital SLR needs to work out the amount of light in the scene you are photographing.
It does this by using a built in light meter. The light meter reads the amount of light in the scene and performs some calculations that average out the light across the area being metered. Once this calculation is complete it decides how long the shutter should be open for, how large the aperture should be and what ISO rating should be used. These settings are used to determine what the camera believes is the ideal exposure for the scene.
The problem is what the camera thinks is the ideal exposure and what you think is the ideal exposure may differ. Sometimes they differ by a long way.
To help you and your Digital SLR get closer to agreeing on the perfect exposure your camera will have different types of light metering.
To confuse matters slightly different brands call the different types of metering by different names.
Here is a list of the different types of metering you might come across:
For good all-round results in everyday shooting situations, cameras use a system that involves breaking the scene down into different zones or segments and analysing what each contains, averaging the results to give a good overall exposure. Different manufacturers have different names for this metering system. Nikon call it Matrix, for example, whilst Canon call their system Evaluative and Sony call it Multi-Segment.
For more accurate metering, look for a camera with more metering zones. The Pentax K-r uses only 16 zones, compared to 77 on the more expensive K-5. Canon's older entry-level models, such as the EOS 1000D or the EOS 500D only had 35 zones, for example, while newer models such as the 1100D or the 550D have 63 zones. Sony's Alpha 350 has 40 zones.
Other metering modes include Partial. Partial looks at the central part of the scene in a similar way to Centre Weighted metering, but the area it takes its reading from is much smaller.
The central portion of your scene is often where you will position your main subject and therefore is the most important part of the picture to get correctly exposed. So you may decide to use Partial metering when a relatively small, central area of the shot is either much lighter or darker than the rest of the scene.
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This guide was written by Ian Younger
Matrix metering is a relatively new addition to camera technology. Before its introduction Centre Weighted metering was the standard default type of camera metering. It took its reading from a large area in the centre of the scene. Although Matrix metering is capable of making a more educated guess than Centre Weighted metering it is also more prone to error. Therefore some photographers prefer to use the more simplistic Centre Weighted metering.
Even if you do not use it as your default metering mode it can be worth switching to Centre Weighted if you find Matrix struggling to produce the results you want for a specific type of scene.
An example of where Centre Weighted metering often gives more accurate results than Matrix metering is in scenes with a high dynamic range.
Spot Metering allows a light reading to be taken from a specific point in the scene. It is similar to Partial metering, but the area it meters is a lot smaller and more concentrated. This can give you an extremely accurate light reading.
For example, a wedding photographer may take a Spot meter reading from an area of the white wedding dress. If Matrix metering was used the camera would average out the light across the scene and the dress would look grey. By taking reading at a specific point on the dress its will appear white in the photograph.
In a similar way, a portrait photographer will take a reading from the subject's face to ensure good skin tones.
Spot metering usually involves pointing the centre spot of the viewfinder at the area that you want to measure the light from and then pressing a button to lock that reading in before then re-composing and taking your shot.