What is Metering?
Metering is how your camera determines what the correct shutter speed and aperture should be, depending on the amount of light that goes into the camera and the ISO. Metering modes are the methods by which the camera works out the available light.
Metering works by assessing the amount of light available for a photograph and then adjusting the exposure accordingly.
Most of the time the default setting works well, however, sometimes the camera isn't intuitive enough to get the exposure right when using Program, Shutter Priority, or Aperture Priority modes. In these cases, you can make manual adjustments to the metering mode used by the camera. If you are in full auto mode then you generally cannot change metering modes.
Every DSLR has an integrated light meter that automatically measures the reflected light and determines the optimal exposure. Metering modes change the area and priority the light meter works on.
Let's take a closer look at the three main metering modes that both Canon and Nikon cameras have. We'll see how they meter the light and when you should use that mode.
In the sections below you will find for each metering mode, the icon commonly used and a representation of where the metering occurs within the frame. The red area is the area being metered, with bright red having the most priority and light red/pink having minimal priority.
Evaluative (Canon) / Matrix Metering (Nikon)
Evaluative metering is the default metering mode and the only one used in automatic modes. Evaluative metering is best to use when lighting is relatively even across a scene, such as in a landscape with no extreme front or backlighting, and is also good for most sports photography.
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|Evaluative metering takes all the pixels present in the entire scene tries to average the overall brightness and chooses the exposure accordingly.|
This mode is a good all-rounder and is probably the easiest to use, but the downside is, it gives you the least control. I leave my camera metering mode on evaluative metering for most of my photography needs and only change it if the camera is struggling with the scene.
In centre-weighted metering mode, the camera gives more priority to the tones in the centre of the image although it will also take some information from the outer meter points. It's easier to understand how the camera will react using this mode but the area it meters for is still quite large, so whilst it will give a better result in some situations, it's not the ideal mode.
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|Centre-Weighted still looks at the entire frame, however the central part of the view is given priority. and the corners are almost ignored.|
Unlike evaluative metering, centre-weighted metering does not look at the focus point you select and only evaluates the middle area of the image.
Use this mode when you want the camera to prioritize the middle of the frame, which works great for close-up portraits and relatively large subjects that are in the middle of the frame.
With Spot Metering, the camera meters only a small part of the frame. This means that you can take a very precise reading, getting the exposure correct for your subject, and ignoring brighter or darker areas in the rest of the frame.
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|Spot metering only uses a tiny fraction of the central point to evaluate metering.|
Spot metering works great for back-lit subjects which would otherwise appear as silhouettes. Spot metering works well with exposure lock which we will look at in a few minutes.
Partial Metering (Canon)
Canon cameras have an additional metering mode called Partial Metering. This mode functions a lot like spot metering, but the area the camera meters is a little bit larger.
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|Partial metering uses the a larger central area to evaluate metering.|
Usage for partial metering is the same as spot metering.
Exposure Lock, or AE-Lock, is a small button located on the back of the camera body which most people have never pressed. This button allows you to lock in an exposure value until you press the shutter.
On Canon cameras, the AE-Lock button is indicated with an asterisk (*) whilst Nikon uses AE-L as a label.
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Normally when you move the camera around it is constantly metering the scene and adjusting the settings. You can see this by watching the aperture and shutter speed and pointing the camera from light to dark areas. You'll see these values adjust for the correct exposure.
You can also activate AE Lock by pressing the shutter button halfway down. This will also lock in focus and start image stabilisation as well.
AE-Lock is very useful when you use spot or partial metering. The centre AF point takes a meter of your subject and focusses on it. The problem is you want to reframe your photo to place the subject off centre. As soon as you move away from the subject the camera re-meters the scene and now your exposure is off. When you press the AE-Lock button, it freezes whichever exposure parameter was set by your camera until you take a picture. Now you can move around and reframe your shot without affecting exposure.
Exposure compensation is used to alter the exposure from the value selected by the camera, making photographs brighter or darker. While the metering modes work quite well in most cases, you might experience overexposure or underexposure in more challenging lighting conditions. Photographing in strong shadows, or with a backlight, the camera meter might be adjusting the exposure too aggressively. This is where Exposure Compensation comes into play and you can manually take control of the brightness using the exposure compensation feature.
The method by which you adjust the exposure compensation varies by camera make and model. While most cameras will have a dedicated button on either top or the back of the camera, some cameras might have this feature available only through a dial or a menu screen. In any case, look for the plus/minus icon or consult your manual for further instructions.
The steps here are for Canon DSLR's, however, once you find the method for your camera the principal is the same.
The next thing you need to locate is the exposure meter in the camera's viewfinder or screen. This usually looks like a scale between -3 and +3 with indicator markers which you can see at the bottom middle of the photo below.
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On Canon cameras, the exposure is adjusted by half depressing the shutter then scrolling the command wheel on the back. As you start making adjustments to your exposure through exposure compensation, you will notice a bar going to the left or to the right of the middle 0 value, which indicates that you are dialling in negative or positive exposure compensation.
Exposure bracketing is a technique in which a series of photos are taken, one underexposed, one normally exposed and a third overexposed. Exposure bracketing is useful for tricky lighting conditions where you need shadows and light areas to be exposed well without the other being clipped. An example of this is HDR photography, where the camera automatically blends these frames to make one evenly exposed photo. Exposure bracketing takes three photos in quick succession and allows you to blend them in HDR post-processing.
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Experiment with Metering Modes
Metering modes aren't perfect, but they are close. The most important thing when you are shooting is to check your histogram to be sure your exposures are good. If you are shooting in manual mode, you can choose to increase or decrease your aperture, shutter speed, or ISO based on the results you are getting. If you are shooting in aperture or shutter priority mode, you can use exposure compensation to tweak your exposure.
When conditions allow, changing between metering modes where possible, allows you to experience the differences between each setting yourself. There is nothing like knowing in advance how different lighting situations can be handled in camera.