August 14 2007
Exposure systems prioritize areas of the scene differently giving a range of metering solutions to meet different lighting conditions. Tips on the correct digital camera exposure pattern.
The assumption that all the photographer has to do is point a digital camera and the automatic programs modes will produce a perfectly exposed and in-focus shots every time is wrong.
Although not highlighted in the promotional material, hidden away in many compact digital cameras is a choice of metering modes. Of course this feature is also standard on high end pro style digital SLR models.
The different modes measure not only the amount of light in a scene, but also where it is and this location influences the final exposure setting. The camera designers include a variety of metering modes because there are so many different scenes that even modern digital cameras do not get it right all the time.
Multi-segment, Matrix, or Pattern Metering
This is the latest metering system that divides the scene into a number of separate segments. The light in each of these segments is measured and then the information from all the segments contributes to the exposure settings.
This takes into account not only the amount of light in each part of the scene, but also the significance of its location in the scene.
For typical scenes that conform to one of the pre-programmed lighting models, the exposure set by the camera should be correct, and for over 90% of the time it usually is.
Center Weighted, or Average Metering
This is the metering method in vogue before multi-segment metering and works on the principle that the area in the center of the image is more critical than the edges. While it provides an average exposure setting based on the overall scene, it gives increasing emphasis to the light at near the center of the image.
This only considers the exposure value of one small spot in the center of the image and ignores the rest of the scene.
The accompanying photos are a series using the different metering patterns. All used shutter priority mode, with the camera’s light meter deciding on the lens aperture, based on one the three metering patterns.
In the first photo, the camera chose an aperture of f16 based on multi segment metering. However, the clock tower is underexposed because the large areas of bright blue sky in the background did not match the camera’s programming. The histogram of this image from the Gimp shows a large area devoid of pixels on the right, suggesting underexposure.
The second shot used center weighted metering and the result is an even more under exposed image; the camera selected a smaller aperture of f19. A contributing factor is that in the composition the tower is left of center, so part of the center is bright blue sky so that even though the subject is the clock tower. Even though it is the subject, because it is not dead center then it is less important in determining exposure than some of the background.
The third picture used spot metering and the meter reading of f9.5 came from the sandstone of the tower, producing an accurate representation of the scene’s brightness.
If the composition of the picture requires the subject to be off center then take a meter reading using the center spot on the subject and lock that exposure , usually by pressing a camera control, then compose with the subject in the desired position.
If your digital camera does not have all this choice of metering patterns a way around is to try and isolate the subject, getting in close, and get a camera meter reading and then use exposure lock keep this setting and then compose and shoot. This is effectively the same as spot metering.