A light meter is a device used to measure the amount of light. Typically, a light meter will include a digital or analog electronic circuit, which allows the photographer to determine which shutter speed and f-number should be selected for optimum exposure, given a specific lighting situation and film speed.
Regardless of how you shoot, and whichever shooting mode you prefer to use, there is one item that remains constant – the light meter. Somehow, either you or your camera must know how much light is on your scene to determine the optimal combination of aperture size, shutter speed, and ISO sensitivity to get the photo you want. This tool, which may not seem relevant to new photographers, is called a light meter.
Understanding what your camera's light meter does and how it works is critical to advancing your skills and getting the pictures you want. Hopefully, this article will help you get a grip on it and how the camera light meter works.
Measuring the Incoming Light
When you point your camera at a scene you also need a way of measuring the incoming light so you know how much of it there is and what settings you (or your camera) need to control to get the desired outcome. It is just like measuring the temperature of your food with a thermometer to make sure it is done properly.
Most cameras today use a process called TTL Metering, which stands for through-the-lens metering. It means that your camera examines the light coming in through the lens and evaluates the scene's brightness. You, or your camera, can adjust the settings to make sure your photo is exposed to how you want. You may not even notice the light meter at work or even see that it is there unless you shoot in Manual Mode. But trust me, it is continuously monitoring the light whether you know it's working or not.
Matrix metering, which is also known as evaluative metering on Canon cameras, and by yet still more different names by other camera manufacturers, is the default setting on most DSLR's. It divides the frame up into separate zones, and analyses all the light data available in these zones. By reading this information, it aims to produce a balanced exposure over the whole scene.
Unlike the other two modes, matrix/evaluative uses the whole frame to determine how it will suggest optimum exposure. Like spot metering, matrix/evaluative is affected by the focus point your camera is set the system reads the information from all the zones, then checks where you have set focus it then marks that area as one of the most important compared to the other zones.
Center-weighted metering uses the center of the frame to base the exposure on it evaluates the light in the middle of the frame and surroundings, ignoring the corners. It doesn't change position when you change focus points, unlike spot metering, so it will always base its reading on the center of the image.
Center-weighted metering works well when you want to utilize the center of the frame, such as in a close-up portrait or abstract image. If your subject is backlit like the example used with spot metering above, this metering mode will expose the center of your image correctly, but the background will probably be horrendously overexposed.
Spot metering, as the name suggests, only evaluates the light around your focus point and nothing else. It measures a single, small zone and calculates the correct exposure measured in this area and nothing else.
If your subject doesn't take up much space in the frame and is back-lit, spot metering is the best mode to choose for this scenario. The exposure is worked out from your focus point, and it will ignore the bright light behind your subject, even on a sunny day. If you took the same shot with center-weighted or matrix metering modes, your subject would probably come out as a silhouette.
You can also use spot metering for a bright subject against a dark background, such as the moon in the night sky. It will only measure the brightness of the moon, not the sky around it.