How Exposure in Photography Works

Measuring the Light and Dark in Pictures

Exposure is the amount of light that hits the sensor and creates an image. If a subject is over exposed, there is too much light, if it is underexposed there's not enough light.

If the subject is correctly exposed then the right amount of light has reached the sensor, although In photography the "right" amount of exposure is subjective.

Multiple variables effect the exposure, how light or dark the subject is, its texture, and whether the photographer is using a Telephoto or Wide-Angle lens. Determining the "correct" exposure will take time, and an understanding of equipment and experience.

How Exposure is Measured

Light meters are the primary tool for measuring exposure. These meters come in multiple forms but all work in one of two ways : reflective metering, and incident metering.

Reflective metering, either in-camera or hand-held, is where the meter calculates the amount of light that bounces or "reflects" off the subject and back into the sensor.

Incident metering by a hand-held light meter calculates the amount of light that strikes the subject.

Most photographers have put aside the time-consuming and cumbersome hand-held light meters in favor of utilizing in-camera meters. The in-camera meter in most cameras will have additional options that can be configured to your style and preference, such as a center-weighted meter, spot meter, and multi-segment meter.

The center-weighted meter views the whole picture and calculates the exposure, giving more "weight" to the area in the center.

The spot meter, uses the information from a specific location or "spot" indicated by the photographer to calculate the amount of exposure required.

The multi-segment meter operates much like the spot meter, but instead of one spot or area, it utilizes multiple areas of the picture to calculate the correct exposure.

How is Exposure Controlled?

There are three determining factors when controlling exposure.

• ISO - the sensitivity of the light meter
• Shutter Speed - the amount of time the shutter is open
• Aperture - the "iris" of the lens