Controlling the exposure of a centre-weighted meter
(Tuesday, March 15, 2005)
The vast majority of compact digital camera use a centre-weighted metering system. That means that a greater importance is given to a central area of the frame than to the surrounding part.
This is not always the best, since the subject may not fall in that precise location. In such circumstances, the manual usually instructs the user to first focus the camera on the subject, then re-frame. Most cameras use a two-stage shutter release, where the first half-squeeze sets the autofocus and the auto-exposure, and then takes the photo when the shutter release is pressed in all the way. The result of this system is that whatever is placed at the centre of the frame, and focused on, is also at the centre of the light metering pattern. And, therefore has great impact on the overall exposure.
In some situations, the results can be less than perfect, with the subject well-exposed and focused, but with the rest of the frame either over or under exposed. The result can be even less satisfactory if the image has been recomposed after the autofocus (AF) and the auto exposure (AE) has been done.
To illustrate the point, we can look to the photos below.
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The images above were taken seconds apart. For the one on the left, the photographer decided the composition desired and took the photo. Since the metering, along with the autofocus was done in the centre of the frame, the image is sharp and the foreground correctly exposed. However, an interesting element of the photo, the green roof of the building in the background, is quite overexposed. The reason is simple: the camera's meter evaluated the scene based on a darker, more light absorbing subject: the tree in the foreground. Since the metering gave more importance to the light coming from that part of the frame, the background became a bit overexposed, and the green colour of the roof almost disappeared.
To take the photo on the right, the auto-exposure/autofocus was done by pointing the camera at the branch and sky in the upper left-side. Then, with the shutter release held halfway, the image was recomposed to be similar to the one on the left. Since the area evaluated by the camera contained a better combination of "tree" and "sky", the result is an image that has a better overall exposure. The result can be seen with the building's roof, which is now green, and contributes to the interest of the image.
Posted in Posted by cheewooi at 6:49 AM
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