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The Almanac - Metering Modes in Digital Cameras

Published by Hari Kumar Balasundaram under , , , , on 2:18 PM

After a brief hiatus, The Almanac is back with basics in metering again. This time though we take a peek into the world of digital cameras & DSLR’s & their inbuilt modes for metering the correct exposure for every picture shot.

So what happens when you look through the viewfinder and decide to take a picture, many cameras require you to first press the shutter button halfway down. This allows the camera to properly focus. To capture the shot, typically requires the shutter button to be fully pressed.

Between the time you first press the shutter halfway and the camera takes the shot, your digital camera's brain does the following...
  1. Determines what the subject of your shot is
  2. Focuses the lens
  3. Figures out how much light there is for your subject
  4. Calculates the correct combination of shutter speed (how long the shutter will remain open) and F-stop (size of the aperture) to properly expose your photograph

But the most impressive thing is that your camera accomplishes all of these feats in a fraction of a second. In terms of photographic basics, when your camera "meters" the scene, it is calculating the correct shutter speed and F-stop, based on the light at your subject.

The three most common metering modes that are available on most digital cameras are the Matrix Metering Mode, the Centre Weighted Metering Mode, and the Spot Metering Mode. Each of these metering modes have their strengths and weaknesses and the key to making great pictures is understanding those strength and weaknesses because only through understanding them will you know which mode to use in a given shooting situation.

These common metering modes are available on all DSLRs but the way you access these menu items vary from camera to camera so once again dig out the manual that came with your camera and read up on its metering modes and how to access them.

Matrix Metering
Matrix Metering Mode, in this mode the camera divides each scene up into a number of discrete segments, the numbers of discrete segments vary from camera to camera. When set for matrix metering the camera takes separate reflected light readings from each of the zones in the matrix. The camera then selects an exposure setting that is best for the majority of the frame zones sampled. Matrix metering, also known as multi-segment metering or multi-pattern metering, is the best metering mode to use most of the time because it assures you the best overall exposures under most conditions.

Centre-Weighted Average Metering
Centre-weighted average metering gives special emphasis to the centre of the frame, but also covers the surrounding area. Some photographers prefer this metering mode because it’s simpler and more predictable than multi-segment metering, which uses complex algorithms to sort through the data from all the different points of measurement.

Centre-weighted metering is a good mode to select when you want to capture a brightly lit subject and be sure it’s exposed correctly compared with the surrounding areas. This is an excellent compositional technique because people’s eyes are always drawn to the brightest part of a photograph.

Unlike multi-segment / matrix metering, which interprets virtually the entire picture area, centre-weighted metering does not attempt to identify and correct situations like backlighting. This means you have to be a bit more careful when using this type of metering. However, many experienced photographers prefer this mode because they can apply their own exposure compensation (+ and –) and know that the camera hasn’t automatically tried to do likewise.


Spot Metering
Spot metering measures only a small area in the centre of the frame. Typically, this area is a spot that’s only about two or three percent of the entire area of the picture (hence the term “spot” metering). When you have a select area of a picture that you want to precisely meter, and don’t want other areas of the scene to affect your exposure. However, it can be a challenging metering mode to use because you really have to pay attention to exactly what it is measuring and interpret the brightness of the spot yourself. The size of the spot varies from camera to camera, with high-end cameras usually having a smaller spot than entry-level cameras.

Partial metering
Not all cameras have spot-metering mode. For those that don’t, they’ll typically offer a partial-metering mode instead. Partial Metering covers an area exclusively at the centre of the scene, taking up about 10 percent of the total picture area—not quite as small as that of a spot meter. Here, too, if the surrounding area is darker or lighter than the main subject, this mode is a good choice. It will usually give you the correct exposure for your subject, as long as your subject isn’t very light or very dark.


Now that you know what your camera does when metering a subject, May use the light to good use and come back with perfectly exposed photographs.

Until then – Happy Clicking Mavericks!!!

Hari Kumar Balasundaram
[Content sourced from various blogs]


2 comments:

Ashok said... @ November 20, 2009 at 3:33 PM

Very Useful information which I was looking for.... Thanks a lot Hari....

Rathna said... @ March 18, 2011 at 3:43 PM

Hari,
Thanks for the info! Oru kelvi - I am able to comprehend how metering might run its algorithms and adjust exposure when am on Auto mode. But if am on Manual shooting mode, and set my aperture to one value, would the metering still work automatically or should I set the metering level or something manually?

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