Using Light Metering Modes To Get Camera Exposure
Controlling exposure is an important part of the image creation process in digital photography. This is seldom emphasized enough, but it should be explored, since it is not often taught as part of the basics. Metering determines the ideal settings for a proper exposure when shooting in automatic mode or any of the priority modes (e.g. shutter and aperture). In the old days of photography, hand-held light meters were used by photographers to determine “correct” exposure settings based on the amount of light the meter records. I state it as “correct” in the context of the light meter, since some photographers would have their own interpretation of correct exposure. These light meters were needed to give the photographer an idea about the lighting conditions since they shot in film and could not preview the image after each shot.
Today, digital cameras (DSLR and mirrorless) have a built-in integrated light meter, so an external hand-held light meter is not so much required. Beginning photographers may not be aware of this feature that well, but you can see the meter in action from the preview display screen on the back of the camera. The in-camera light meter, also called the exposure meter, reads the reflected light from the subject. This measures the optimal exposure based on the reading “through the lens” (TTL). Some photographers still use hand-held light meters to determine camera exposure settings, but is not required for everyone. The integrated light meter in the camera should be sufficient in most situations.
Digital cameras have a special metering sensor that provides this feature. Canon was one of the first (if not the first) camera makers to incorporate metering sensors in their cameras. It uses a microprocessing chip (an IC or integrated circuit) to calculate the brightness of the light that is reflected from the subject. This provides the camera settings for the exposure, mainly the aperture and shutter speed. The ISO can also be included in the settings, but has little effect with ISO invariant cameras.
Calculating The Exposure
The common formula for reflective meters can be expressed as:
- N is the relative aperture (The F-stop)
- t is the shutter speed (Exposure time)
- L is the average scene luminance
- S is the ISO (In terms of speed for film and brightness with digital sensors)
- K is the reflected-light meter calibration constant
This is the square of the aperture over time is equivalent to the product of luminance and ISO over a constant K.
For the constant value of K, ISO 2720:1974 recommends a range of 10.6 to 13.4 with luminance in cd/m². 12.5 is used by Canon and Nikon, while 14 is used by Minolta and Pentax to give some examples.
According to Nikon:
“The camera optimizes exposure by adjusting shutter speed, aperture (f-number), and ISO sensitivity according to the brightness of the subject, which is measured using the camera’s built-in metering sensor.”
In the camera preview screen (located on the back of the camera), there is an indicator for the exposure meter. This looks like bar with numbers going from -2 to 0 on the left side and 0 to 2 on the right hand side. This is the meter reading that gives the photographer information about the exposure.
When the reading is less than 0 (shifting toward the left), the image is being more underexposed or darker. When the reading indicates greater than 0, then the image is being overexposed or lighter. The more acceptable value is for the meter to be as close to 0 for the proper camera exposure setting.
The light meter indicator appears differently, depending on the the camera brand. Canon and Nikon place the exposure meter along with the camera settings for shutter speed and F-stop value. The camera also provides exposure compensation to allow photographers to customize and adjust the exposure. A +/- button (depends on the camera) allows this adjustment to be made. A “+” indicates a lighter exposure, while a “-” indicates a darker exposure. These are in units of EV, which are one exposure step or stop per unit.
Types Of Metering Modes
There are three main types of metering mode methods. The terminologies may vary among camera makers, so check with your documentation for more information. The main types will be discussed from a Nikon terminology.
In Canon terminology, this is also called Evaluative Metering. In this method, the camera measures the light falling on the subject by dividing the frame into zones. It then calculates the exposure setting based on where the focus was set. This type of metering ensures a balance in the dark and light areas of the scene. It also uses data of the light coming from the whole frame to get that balance. This mode works well for most photographers, ideally for shooting landscapes, group shots and simple portraits.
This method sets the metering at a very small area in the frame. It evaluates the light around the focus point set by the photographer and ignores the rest of the frame. This is about 3.5% of the area where the focus is set. This mode is very good for brightening shadows in portraits, especially when the scene is back lit. An example is when you are shooting a subject against the light. Often the background is blown out and the subject is underexposed. Using spot metering, the photographer can ignore the background and just meter the light to get an exposure on the subject. We are only looking at the light coming from the subject to get the best exposure setting. Canon provides another type of metering method called Partial Metering, which is similar to Spot Metering, but covers a much larger area (8% of the area).
Center-Weighted Average Metering
When you want to measure the light from the center of the frame of your subject, this is Center-Weighted or Center-Weighted Average Metering. The exposure is set to the light coming from the center of the frame and does not rely on the focus point. It also ignores the light coming from the corners of the frame. This is ideal when shooting landscapes, macro shots and close-up portraits where the emphasis of the image is toward the center of the frame.
Issues With Light Metering
One of the issues faced with light meters are the varying levels and intensities in lighting conditions. This can at times throw off the light meter values and give a less than optimal result. The light meter works best in a well lit environment where the light is more even on the subject. This can mean the difference between a good and a bad exposure.
As an example, let us say you are taking a image of the subject in a snowy area. The light meter will try to balance out the subject from the background to a gray level. It could brighten the white snowy background and underexpose the subject if the meter was not correctly specified. This might throw off the meter because of the different light intensities.
What the meter is trying to do is balance out the dark and bright parts of the scene the photographer is capturing. A bright blue sky with no clouds can result in a good exposure. Add a subject (person) and clouds to the scene, things change. Now the meter will have to determine how to expose the clouds and the subject as well. In this case the photographer will have to determine the best metering type if they are trying to capture the whole scene in the frame or if it is just more about the subject in a portrait with shallow depth-of-field.
Another thing to take note of is that metering modes allows the camera to set the exposure using automatic or semi-automatic shooting modes (Shutter and Aperture Priority). Off-camera, hand-held light meters can help photographers shooting in manual mode by providing them exposure settings. This lets the photographer either use those settings or set a custom exposure based on that information.
Photographers can use the metering mode to get the optimal settings for an exposure. When shooting in automatic mode, programmed mode, aperture priority or shutter priority mode, using the metering feature will provide optimal results. It works well in well lit conditions, but can be less than accurate when there are varying light intensities. In that case, photographers will have to rely on their experience or use a hand-held light meter (external). That means setting the exposure manually.
Photographers can take control of the entire exposure process (e.g. shutter, aperture, ISO) in manual mode. Does that mean that metering becomes unnecessary when shooting manual? Not really, but in this case it can still be used as a guide to the photographer. By looking at the indicator on the back of the camera, the photographer can still see how close the exposure is to 0 based on the metering sensor’s readings. Therefore it still has use for photographers who shoot in manual mode, allowing them to adjust their exposure to get as close to what the meter indicates. This can be verified after the image is captured by checking the histogram.
The meter indicators can be simply ignored or used to get more information about a scene the photographer is trying to capture. As photographers advance, they begin to shoot more in manual mode as part of the creative process so the metering can be ignored. There are still times when it is useful, like in uncertain lighting conditions. Just be careful though, because the metering sensor can be fooled by scenes of varying light intensities or tonalities.