We have all heard the comment, “well I just fix it in Photoshop™”. I’m not too sure about you but I would rather spend my time in the field shooting than in front of the computer fixing images. One of the best pieces of advice I ever heard, whether shooting film of digital is “get the exposure correct when you take the picture”. If you do this, you will not need to “fix” it later. Over the next few months, the digital corner will go back to exposure and metering basics. This month we’ll look at the metering modes available in today’s digital SLR camera bodies.

Most digital SLR’s have followed their film predecessors in having a variety of metering modes. The 3 basic ones are matrix or pattern, center weighted and spot. In the case of my Canon 40D, the engineers have added a 4th they call “partial”. This forth one is really a variation on the spot meter capability.

The “meter” in the camera is really a combination of the light sensors and the computer’s microprocessor. The processor takes input from the sensors and computes the exposure based on the sensor light readings, the aperture setting, shutter speed, focal length, and a lot of data stored in “look up tables”. Let’s look at each of selectable metering modes of the sensor and consider how to use them most effectively.

Matrix or pattern looks at the entire image divided into a number of segments. In the case of the Canon 40D, there are 35 segments or zones evaluated. Here is an example of the zones in a simpler, 13 segment matrix pattern:

As you can see, the zones are not uniform, but are designed to give the camera’s processor information according to a “best case” for normal compositions. Normal is defined as average distribution of light, medium and dark tones. Matrix is usually good for front lit scenes, or scenes with minimal contrast or a moderate mix of light and dark tones.

Center weighted and partial modes look at the entire scene but give substantially more “weight” to the center. Typically 70% of the information from the center area and 30% from the periphery are used to calculate exposure. Here is an example of the center weighting approach.

Center weighted exposure is probably one of the better tools we have. This mode uses a lot of the experience developed over the years of film shooting. Center weighting, along with the exposure lock feature on the camera, provides a very good tool for getting the best exposure possible. This mode is best for scenes with highly directional light, a scene with very bright sections and very dark sections, like a landscape with bright sky and dark foreground, and high contrast scenes. To most effectively use center weighting, while controlling over exposure, always include the brightest area in the center (biasing toward the highlights). Take the reading in manual or any of the creative exposure modes (shutter or aperture priority) and lock the exposure with the AE lock. Then recompose and shoot! A word of caution, this mode, like any other, averages to middle grey so you may need to compensate for very white or very black subject matter.

Spot and partial metering looks at a small part in the center of the frame. All exposure information is calculated from that area. This metering mode is very good for subjects that are in shadow, where you need to control or saturate highlights, and high contrast scenes.

Spot metering also averages to middle grey so you may need to compensate or look to use white balance to get a correct color rendition.

Next time you’re out shooting, try these different metering modes, consider the lighting of the scene and take a few notes on which mode you selected for each shot. Consider adding some comments on why you chose that mode, and see how your images come out. Do this a few times and maybe you’ll spend less time in front of the computer and more behind the lens!