A handheld light meter offers many advantages.
Automatic camera-shooting modes solve a number of tricky lighting situations using in-camera meters, but sometimes an automatic mode just doesn't measure up to the situation. These meters are easily fooled by complex lighting situations, distance and conditions. In these cases, a handheld light meter distills a complex situation into a simple calculation for the photographer.
Portability
Sometimes it's just not possible to move your camera, particularly when you are shooting with equipment such as a medium- or large-format camera or have created the perfect portrait setup. Some older cameras do not have the advantage of an internal light meter. In these situations, it is easier to use a handheld light meter to get closer to your subject. This allows you to take a more accurate reading of the light falling on, around or reflecting from your subject without having to compromise your scene.
Accuracy
In-camera meters have an unbeatable convenience, but they are limited by the areas that they cover with their sensors. Situations such as high contrast scenes -- think a bride in white next to her groom in his black tuxedo -- can fool the meter and cause an inconsistent, incorrect meter reading. A handheld meter allows the photographer to meter a number of areas, and more advanced models have the option of creating an average reading from several separate readings. This creates an accurate reading in a situation that confuses lesser meters.
Incident Light
One of the most important features on a handheld meter is the ability to read incident light. In-camera meters measure the light that is reflected from the subject, but this isn't always the right reading for the correct exposure. Measuring the light falling on the subject, known as incident light, creates an accurate reading based on the amount of light in the scene. This avoids confusion in high-contrast scenes entirely, making the reading the most accurate possible.
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