The camera’s light meter indicator presents exposure information in a simple, easy to understand way that helps you to quickly know if your exposure settings will produce a properly exposed image. Pictured below, the light meter indicator is usually accompanied by exposure settings for ISO, Aperture, and Shutter Speed.
The light meter scale is measured in Stops; usually in a range from -2 to +2 full Stops, but is able to be read in 1/3 Stop increments. Some current digital cameras display +/- 3 Stops.
When the light meter indicator is reading Zero (0) the image is perfectly exposed (according to the camera). When showing a negative value the image is under-exposed (too dark), while a positive value means the image is over-exposed (too light).
Most of the time the light meter will be reading at or next to zero. For all the the pre-set “Scene” shooting modes. and the Automatic modes (Full Auto, and Program) the camera determines the exposure values, and *should* compensate accordingly to result in a perfect exposure. (i.e. light meter scale reads zero). When shooting Aperture Priority, or Shutter Priority, the camera should still be able to make a perfect exposure by complementing your user controlled setting (e.g. aperture, or shutter speed) with an appropriate balance of the other exposure element.
In Manual Mode, the user sets all readings. This shooting mode relies most often on the light meter readings. For all other settings, the *only* time the camera light meter *should* be reading under, or over-exposure to an amount greater than +/-1 full stop is when there is too much light (e.g. ISO is set too high) or not enough light (e.g. dark conditions).
Its a good habit to be in to check your light meter reading before taking the image; just to be sure the exposure appears to be correct. It is important to note that some lighting situations will trick your camera in to thinking enough light exists to take the image, but the result is a dark image. This can often happen with shots into the sun, or in scenes where there are both bright, and dark portions in the scene. Similarly, in Manual Mode, there may be times when your camera believes there is not enough light to take the image, though the results are fine. This can happen especially when shooting fast shutter speeds, and/or using spot or partial metering when the camera is measuring a dark object.
As a final note, understanding your light meter reading is a basic skill all photographers should do. Be also aware that you can easily adjust the camera’s exposure settings (ISO, shutter speed, and aperture) by using the exposure compensation button. For more on adjusting the exposure, click the following link: EXPOSURE COMPENSATION