It’s important to know just how your digital camera light meter works if you want properly exposed images. The metering system more or less controls the camera’s aperture and shutter speed settings and it does this based on the ISO speed and the image’s lighting conditions. The metering usually includes spot, evaluative zone or matrix, partial, and center-weighted metering. All of these have lighting conditions that they excel at as well as those they fail at. It will help your end results if you understand these options.
Incident and reflected light
All light meters in cameras are a little flawed from the start since reflected light is the only type they can measure. This means the camera has to guess how much actual light is shining on your subject. If each object reflected the exact same amount of incident light it wouldn’t pose a problem. However, all objects reflect different amounts of light. Due to this, camera light meters are standardized and they base their metering on how much light a middle gray object would reflect. When the camera is pointed at something that’s lighter or darker than this shade of gray the meter can’t properly calculate how much light is being reflected and this will result in images that are over or underexposed.
Middle gray is typically an 18 per cent reflection of incident light, but most cameras figure it out to be anywhere between 10 and 18 percent. If the object in the image reflects less or more light then the exposure will likely be off. However, even if the photo has dark and light objects in it, the average amount of reflected light could fall into the camera’s middle gray range. If your image is of a black cat sitting on a black rug or a white object in the snow then you may have a problem getting the correct exposure.
If you want properly exposed shots for a wide range of reflectance and lighting options you can try and meter different areas of the scene. The whitest parts of a scene are used the most for exposure calculation while black sections are basically ignored. The best way to meter the image is to calculate the reflected light for each part of the photo. This means the scene is divided into sections and you can calculate the amount of light for each of them.
Partial and spot metering
These types of metering will allow you to control the exposure since you can select the part of the image you want to meter. However, it takes a bit of practice to get used to them. With partial metering, it would be useful when taking a shot of somebody who is backlit. If you meter off of their face it will make sure they person doesn’t turn out underexposed in front of a bright background as this would make them appear to be a silhouette. If the person’s skin shade is a long way from middle gray though, it might not be perfect. Spot metering is mainly used when you need to meter a small or specific area of a scene.
Center-weighted metering and exposure compensation
Center-weighted metering used to be quite common as a camera’s default setting. But these days, matrix and evaluative are more common, especially partial and spot metering. Center-weighted metering will produce predictable results while evaluative and matrix modes use complicated algorithms that are more difficult to predict.
You can use exposure compensation with any light metering mode. This will allow you to change the settings if your photos are consistently under or overexposed. For instance, the white object in the snow as mentioned above would result in an underexposed shot unless you manually adjusted the exposure compensation to plus 1.