The Canon S1iS can meter the light six different ways. Which method is best? Members of the Digital Photography Review were debating that a couple weeks ago.
Center-weighted Average measures the light over the entire frame, but places the emphasis on the centre (inside the green rectangle). On the Canon S1is, the icon looks like [ ].
Evaluative Metering breaks the image into sections, and measure the light in each section. The camera then applies some sort of formula to figure out the exposure and aperture settings. The problem is that we don't know the formula. On the Canon S1is, the icon looks like [(o)]. Evaluative is used by all of the special modes, such as sports, stichted, and movie.
As jvkelley explains, "With evaluative metering the camera tries to figure out where the main subject is in the frame. Once it figures that out, it gives that area the most weight when calculating the exposure."
For some camera users, they cannot tell a difference between center-average and evaluative.
Spot Metering measures the light at the center of the frame, within the green rectangle that appears. This method tends to result in the wrong exposure -- unless that's what you want. It is best when the other two methods don't give a satisfactory result, such as a heavily backlit head shot, or pictures of the moon. On the Canon S1is, the icon looks like [o].
As Costas Vlachos emphasizes: "Spot metering only cares about the little square that you see on the screen. It will correctly expose ONLY for that part of the image and nothing else!"
If the part of the image is too bright or too dark, move the green rectangle to the boundary of a bright/dark area. To move the green rectangle, you can either:
* Move the camera.
* Press the Set button, and then use the four-way controller to move the rectangle to 276 different positions in the viewfinder.
Exposure Lock works like this: set the exposure by focussing on a neutral object, and then lock the exposure by pressing the shutter half-way. (The asterisk in the viewfinder indicates the exposure is locked.) Move the camera to take in the subject of interest, and then press the shutter the rest of the way.
Manual Metering means that you set the exposure and aperture. You can do this either by guessing at the settings, or setting the exposure you read off the screen from the Exposure Lock method, or by using an external light meter.
Bracketed Expsosures means you ask the camera to take three pictures: one darker, one at the current setting, and one too light. On the Canon S1iS, press the Function button, scroll down to BKT-Off, and then scoll over to AEB. Press Set, and then move the 4-way controller to spread apart the three green bars. When done, press Set and Function to escape the menu. When you now press the shutter, the camera take three pictures.
In all of this remember that digital cameras do better with underlit scenes; overlit scenes lose detail. You an always use Picasa 2's Fill Light option to lighten up photos that are too dark.
The last page of the Canon S1is manual has an overview of the light metering systems, their icons, which modes can employ them, and the page reference for more details.