In photography, Metering is the method to calculate the optimal exposure for the subject on the basis of information that the camera gets from the varieties of available light sources. Back in old days, photographers used to rely on an external light metering device because cameras were not equipped with the metering sensor that can measure the sensitivity of the light and give you the information about the exposure settings. But today, every modern DSLR camera comes with the sensor that meters the light entering the camera sensor through the lens and determines the correct exposure settings. Even though some DSLR brands use different naming convention for the metering mode, overall idea of the metering is the same. In this article, I will be taking a reference of Nikon D90 to explain different metering modes but the technique implies to all DSLR brands and models.
What is the Metering mode?
Metering mode is a different exposure mode available in modern DSLR cameras, which is responsible to deal with the light sources and calculate the optimal exposure for the subject and the overall image. When you frame the scene, the metering mode determines the intensity of the light coming from the scene into the sensor and gives that information to the metering system. If you are shooting in auto exposure mode or one of the semi-auto; A, S or P exposure mode, your camera will try to set the correct exposure automatically by calculating the correct combination of the ISO, shutter speed and the aperture value (F-stop). But, if you are shooting in manual (M) mode, you would take the reading from the camera metering system as a reference whether it is negative (underexposed) or positive (overexposed) and set the exposure values (Aperture, Shutter speed and ISO) manually. When you point the camera towards the scene and if the scene is too bright, the bar (shown inside the red box in the picture below) moves towards the positive side suggesting you to increase the shutter speed or reduce the aperture size by increasing the aperture value to properly expose the scene. And, if the bar moves towards the negative side, it means the image is underexposed and you would need to decrease the shutter speed or open up the aperture by reducing the aperture value.
Most of the time camera metering system does a good job of sensing the light and calculate the correct exposure, but sometimes, it gets deceived by the light source when there is a mixture of dark and bright light sources. The common examples where the metering system might not give you the best exposure calculation might be a sunset with a darker sky and a bright horizon, or a subject under a very bright sunlight etc. Now, lets talk about the different types of the Metering modes available in modern DSLRs.
There are three common Metering Modes available in almost every modern DSLR and they are,
1. Matrix Metering
Nikon DSLRs use different Matrix metering depending on the type of the lens you are using. Nikon D90 uses 3D color matrix metering II with the type G, or D lenses and color matrix metering II (which doesn’t include 3D distance information) with other types of CPU lenses, which is the default setting for the D90. When you look at the LCD screen on the top of the camera body (on the right side where the shutter release button is), the Matrix metering mode is indicated by the black rectangle shape with the dot in the middle.
While metering the scene using the Matrix metering mode, camera divides the entire frame into multiple zones and calculate the exposure based on the colors of the light, distance to the subject (if G, or D lenses were used), color tones etc. Camera collects the information from all the zones but gives the priority to that area where the focus point is on. Matrix metering mode does a pretty good job most of the time and should be used for a day to day photography.
2. Center-Weighted Metering
Center-Weighted metering mode is represented by the little circle with a central dot inside. Like in the Matrix metering mode, it also calculates the exposure from the entire frame but the center area is given more priority (weight) during the calculation. According to the Nikon, it gives 75% of the weight to the center circle whose diameter can be set to 6, 8, or 10-mm in Center-Weighted area from the Custom Settings Menu. As name suggests, Center-Weighted metering mode doesn't rely on your focused area but gives the priority to the center of the frame instead. It is not recommended setting for most of the time and also considered as a least accurate metering method to use because the subject might not be in the center of the frame all the time. It should only be used when you want to emphasize the center of the frame, in portrait shooting for example.
3. Spot Metering
Spot metering mode is indicated by the little circular dot. Unlike other two methods, while metering the light, spot metering only gives the priority to a small area where the current focus point is and ignores everything else inside the frame. According to the Nikon, it meters 3.5mm circle (about 2% of the frame) centered on the active focus area. It is widely used when you would want to give the proper exposure to the focused area no matter how the surrounding would be.
How to change the Metering mode?
It depends on the camera brand and the model, but in case of Nikon D90, there is a dedicated button left to the shutter release button. While the button is being pressed, if you rotate the rear command dial (also known as the main command dial), camera switches the metering mode from one to another. If your camera doesn’t have a dedicated button for the metering, you can go inside the Metering/Exposure menu and adjust the metering mode.