Metering

I am continuing my blogs on photography after a very long break and this time I will talk about another one of the important terms that define a photograph. The term is called metering.

Metering is the way the camera judges the brightness or darkness of the image. When the camera sensor looks at an image, it had to judge how bright the image has to be before attempting to capture it. Another term for this is exposure, as we had seen previously. If you have an image or a scene that is mainly white but has a small dark coloured object of interest in the center, then it is very likely that you will miss the object or you will not see the finer details of that dark object. In the case of a camera, the camera has metered the images for the bright areas and hence loses the details of the dark object. Similarly, if you have a predominantly dark image, such as in a dark room, and a small bright object, such as a bulb, then the camera will meter for the dark areas and the bright object will look washed out or over exposed.

You can understand this by relating to your own experiences in photography. Have you ever tried to take a photograph of the moon on a dark night with a simple point-and-shoot camera without a huge zoom? You would have probably noticed that the moon would appear totally white against the black background and you would have lost all the details on the moon, such as the craters. This is because the image is predominantly dark and the camera processor has been trying to take the photo as brightly as possible and in the process made the bright object even brighter and has hence overexposed it.

You can also imagine how the brain does this metering. Suppose you enter a dark tunnel in a fast moving vehicle. A fast moving vehicle because you don't want to give your brain enough time to adjust gradually, like when you are walking into the tunnel. So, when you enter the tunnel, everything goes dark and you are not able to see anything. This also happens when you walk into a room at night and switch off all light. For an instant you cannot see anything and all goes pitch dark. Slowly, your eyes adjust to the situation as the brain begins to meter the scene for darkness and you are able to see/identify the presence of objects in the dark. In case of the eyes, it more to do with adjusting exposure than metering, as the pupil dilates and allows more light to enter, but there is still a hint of metering here also. A reverse thing happens when you enter from dark into bright light. For an instant, everything appears white till the brain and eye slowly adjust to the new scene and adjust the exposure and metering apropriately.

The metering on the camera is far far more complex than just adjusting to dark and bright scenes and this is beauty of cameras. You can ask the camera to meter to a particular point of interest in the scene before taking the photograph. There are 3 main types of metering - Average or Evaluative metering, Center-weighted metering, Center or Spot metering. The actual names of these metering would be different on different cameras, but the meaning would be very much similar.

Evaluative Metering meters for the entire frame and decides on an exposure that will try to show as much of the image with correct exposure as possible. This is generally recommended for images of landscape and sceneries and scenes.

Center-weighted Metering is similar to the Evaluative Metering but with the exception that it emphasizes the center area a little more. What this means is that it tries to get the area in the center as correctly exposed as possible even though it may be at the cost of the rest of the image. Although the rest of the image will no suffer too much since it is evaluative metering only. Its just that there is extra weightage on the center area.

Spot-Metering is a more specific form of metering where only the center 10%-15% is metered for exposure. This is used in cases where the entire image frame has a wide contrast/brightness range, but you as a photographer want only the center part of the image with the best exposure. Its not that only the center area is exposed correctly, but the entire image is exposed in such a way that the center area is expose correctly. This could mean that the surrounding areas are exposed correctly or is over/under exposed depending on the contrast/brightness details on the image. Spot meter is the most commonly used form of metering during bird photography and other forms of long distance wildlife photography where is subject is there in the center of the image.