Photography is all about capturing light, but how does a digital camera measure and read light? The answer lies in what’s called metering. A camera’s automated modes interprets the lighting scenario through a built-in meter. But, even using manual mode, the camera reads the light and displays a meter to guide the photographer’s exposure decisions.
Capturing an accurate exposure with your digital camera involves both understanding manual modes and understanding metering modes. Here’s what you need to know about metering.
What is a light meter?
If a camera’s lens is the eye, the meter is the brain. A meter measures reflected light, or the amount of light hitting a subject. It’s the meter that determines where to set the shutter speed, aperture and ISO when using auto mode. The camera’s meter also helps determine the settings for shutter priority and aperture priority modes.
In manual mode, the meter doesn’t change any settings. Instead, the meter displays the exposure value, usually at the bottom or the side of the meter. If the settings will render an image too bright (according to what the meter sees), the meter will read towards the plus side. In the image below, the meter is all the way to the “-”, which means the image will be very dark. When photographers change shutter speed, aperture or ISO, they use the meter to guide their decisions to a proper exposure, so it’s not all guesswork.
So, why use manual modes at all? The problem is that the meter only measures light that’s being reflected off an object. However, the color of that object changes just how much light is reflected. Subjects that are lighter in color will trick the meter into thinking the scene is brighter than it is, resulting in an underexposed image, and vice versa.
Typically, an entire scene has enough contrast between colors that the camera chooses an accurate exposure, but that’s not always the case. Snowy scenes tend to throw off the meter, as well as, say a blackbird sitting on blacktop. In images without a lot of contrast in the colors, or that contain overly bright or overly light objects, the meter doesn’t take an accurate reading.
Since a camera doesn’t actually have a brain, it can’t compensate for the color of an object throwing the exposure off in one direction or the other. Manual mode puts the power in the hands of the photographer. Using the light meter as a guide, the photographer can compensate and overexpose (according to that meter) lighter objects.
What are metering modes?
What portion of the image does the meter measure the light from? That’s where metering modes come in. The metering mode determines if the camera is reading the light from the entire scene, or just a portion of it. The exact name of the metering mode varies based on what brand you shoot, but the concept is the same across the board.
In evaluative (Canon) or matrix (Nikon) metering, the camera’s light meter reads the entire scene. Most cameras use this mode by default. The camera splits the frame into several zones, reading the reflected light in each area. The camera then combines the data from all the areas to determine a proper exposure. With this type of measurement, the meter is reading data from the subject, foreground and background, from any objects in the image and also the sky.
Since evaluative metering measures details from the sky too, this metering mode can often be problematic. If the subject is backlit, evaluative metering will create a silhouette. On a bright day, using evaluative metering may mean the sky is overexposed while the landscape is underexposed. In most scenarios, however, evaluative metering produces a pretty good result.
Centre-weighted metering measures only the light that’s in the centre of the image. If you are framing your subject in the centre, with the lighting coming from behind, centre-weighted metering mode will prevent a silhouette, properly exposing for the subject.
Spot metering is similar in that it only measures light from a portion of the frame. However, spot metering measures a much smaller area. And, instead of measuring from the centre, spot metering measures from where your focus point lies. Spot metering is best for photographing backlit subjects that are small, such as birds or a person in the distance. It’s also helpful for determining exposure of a backlit subject that doesn’t occupy the center of the frame.
How do I change the metering mode on my camera?
The setting for adjusting the metering mode varies between different camera models, even from the same manufacturer. For exact instructions, you can check your camera’s manual, but it’s usually pretty easy to find once you understand the terminology.
Look for the term “metering” or one of the specific labels like evaluative, matrix, centre-weighted or spot. Metering modes can sometimes also be indicated with a symbol, like this:
Basic, entry-level cameras often have their metering mode located in the menu. Browse through your menu, looking for the terms or symbols. If your camera has a good assortment of physical controls, there may be a button with the metering mode symbol. Press and hold that button while turning the control dial to flip through the different options.
The camera’s meter measures the available light to either use an automated setting, or, preferably, to guide the photographer when choosing settings in the manual modes. Metering modes determine where the camera measures light, whether through the entire image, or just a portion of it. Evaluative or matrix metering works well most of the time, but centre-weighted and spot metering are best for when the subject is backlit or very small. Understanding metering is essential to capturing the best exposure in each shot.
Looking to sharpen your photography skills? Join our top-rated professional diploma in photography today!