Measuring the Intensity of Light

In order to shoot an image with the correct exposure, you have to know the correct value of the intensity of light. Photographers use light meters to measure the intensity of the reflective light in a scene. Digital cameras have built-in light meters that are very sophisticated and incredibly accurate. However, their accuracy is subjective. The recommended aperture and shutter values are determined by how light falls in the scene and by how the light meter is set.
The camera’s light meter may recommend an aperture and shutter combination that offers a decent exposure. However, it may not give you the perfect exposure because it doesn’t know what you’re photographing. Lightmeters can’t evaluate colors or contrast. They only see luminance, which is the brightness of the reflected light in a scene.

Cameras with sophisticated light meters can be set to meter, or test, specific areas of the scene. Most DSLRs allow you to choose the portion of the viewfinder to meter. These meter settings include, but are not limited to:

 Evaluative: Evaluative metering operates by dividing the frame into several small segments, taking a reading from each individual segment, and processing the average of the total segments to recommend the best exposure value for the overall image.

 Spot: Spot metering operates by metering within a small target area that is usually in the center of the frame. Spot metering is particularly useful when your subject is placed in front of a relatively bright or dark background. Spot metering ensures that you will correctly expose your subject. The drawback is that the background may be incredibly under- or overexposed. This is why you should bracket (shoot multiple exposures of the same image) when shooting in a situation that requires the use of the spot meter. For more information on bracketing, see “Bracketing the Exposure of an Image,” below.

 Center-weighted: When the camera’s light meter is set to center-weighted, the camera measures the light in the entire viewfinder but gives extra emphasis to the center of the frame. This setting is typically used by portrait photographers, because the subject is usually centered and the background isn’t ignored. If the subject moves out of the center of the frame, the meter assumes the background is the correct exposure, leaving your subject incorrectly exposed. It’s important to point out that light meters provide recommendations only. If the details in the highlights of the scene are more valuable to you, you may choose to expose the image shorter than the light meter recommends. Likewise, if the details in the shadows of the scene are of more value, you may choose to expose the image longer than the light meter recommends. It’s your prerogative as a photographer to use the light meter to obtain the best exposure of the scene in your image.

Bracketing the Exposure of an Image : Even careful metering sometimes yields an under- or overexposed image. This is why professional photographers bracket their images, whenever possible, to be absolutely sure they have a correctly exposed image. Bracketing involves taking three shots of the same image based on the aperture and shutter values recommended by the light meter: one shot underexposed one stop, one shot at the recommended exposure, and one shot overexposed one stop. Shooting the image with a range of three exposure stops is the best way to ensure you’ll have a properly exposed image. Note: Most DSLR models have a built-in, automatic exposure-bracketing feature. Refer to your owner’s manual for directions about how to use it.