I've mentioned metering in many of these tutorials, but I have never taken the time to discuss it at length. Metering tells the camera how much light there is in the scene, and from that it works out how long to keep the shutter open to correctly expose the image. Most of the time, the camera's default metering (called matrix metering) works very well. However, there are some scenes (usually when there are very bright or very dark parts of your photo, like the candle photo to the right) where the camera doesn't do a good job. That's when you should choose one of the other metering modes.
Earlier, I told you that the best way to take a picture of a sunset is to get a meter reading of the sky and use it to determine your aperture and shutter speed. But did you know that your camera has a bunch of different light metering modes to help you do this automatically? Each of these can be used to make your automatic exposures much more accurate.
That way, instead of playing with manual all the time (which is still very fun and very necessary), you can rely on automatic when you need it. You are about to find out the secret to getting more consistent lighting with completely automatic settings. Here's how you do it.
How Your Camera Meters A Scene
A long long time ago, photographers used to carry light meters with them to measure the light coming off of their subjects. They would read the light from the meter, get out a big spreadsheet, and then pick their aperture and shutter speeds based on the reading from the meter. It was a long cumbersome process that held up every shoot the photographer did.
We have come a long way since the days of the first light meters. Now every camera has its own light meter that takes readings from inside the lens. These readings are then sent off to a computer program that calculates the ideal aperture and shutter speed for the scene. This is how automatic exposure on your camera works.
Here's where it gets interesting. The different metering modes are simply different computer programs that give weight to different parts of the scene. Most cameras have spot, center-weighted, and matrix metering modes. There are a few others, but these are the dominant ones you will be using most of the time. Just as you would expect, they all have different weightings and should only be used for certain kids of shots.
Center Weighted Metering
The first automatic exposure cameras used center weighted metering because we hadn't developed more advanced computer programs to calculate light levels. Center weighted metering gives the most weight to the center of the photograph while still allowing other surrounding elements to play their part.
Just look at this image. The center of the sphere determines most of the camera's "decision" when it comes to picking the correct aperture and shutter speed combination. As you move further from the center, the other surrounding elements play less and less of a role. Once you reach the black the region, they don't even factor into the calculation at all.
Many photographers like to use center weighted metering because it is very predictable and consistent. They know exactly what they are getting. If they focus on the center of an image, that's the part of the image that will have the most accurate light levels.
Much like center weighted metering, spot metering emphasizes the center of the frame. The only difference is the width of the sample area. Spot metering only emphasizes a small spot in the middle of the frame, discounting all other sections outside of it. Have a look at the graphic and you will get a better idea.
Spot metering is really handy when you only want to meter for a very small section of the photo. It's perfect for indoor objects that are illuminated by outdoor light. Just focus on your subject, half depress the shutter (so your camera doesn't try to adjust the exposure again), and then frame the shot how you want it before snapping the photo.
Here's an example where spot metering works to take a good shot. The photographer set spot metering on the center part of the shot. While there is a bright sun on the top right of the photo, the camera ignored the brightness of the sun and correctly exposed the leaves. If matrix or center weighted metering had been used, the camera would have tried to correctly expose the bright sunlight which would have meant the leaves would have been a lot darker in the shot.
This is the default metering mode on most cameras. It is also the most advanced and least predictable. Matrix metering emphasizes different parts of the photo depending on how light is distributed throughout the scene. It "looks" at different light level profiles, attempts to find one that matches your photo, and then picks an aperture and shutter speed accordingly.
Matrix metering can be extremely sophisticated, and it can be monumentally stupid. It all depends on what you are shooting and how you shoot it. Matrix metering tends to do a little better than center-weighted metering overall, but every once in a while, it just gets the entire scene completely wrong. It is in no way a replacement for solid photographic expertise.
The final method of metering is to do it the old fashioned way. Use a light meter and tell your camera what light level to take the photo at. In my opinion though unless you're shooting in controlled circumstances like at a model shoot for a magazine, it's usually more trouble that it's worth. Instead stick with Spot Metering and point your camera to the part of the photo to expose correctly before repositioning to frame your shot.
Which Metering Mode Do I Choose?
The answer, as always, is 'it depends'. Always pick your metering mode based on the image you are trying to capture. Choose spot metering when there is a color or small section of the photo that you want to emphasize, and use matrix metering for everything else. If matrix metering isn’t working for one particular kind of image, quickly switch to center-weighted metering and see it turns out any better.
With practice, you will learn all of the subtleties that come with these modes. It’s a handy thing to know. Let me know how your experimentation with the different modes goes.
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