Exposure meter or light meter is used for measuring brightness of the light and set the aperture/shutter speed settings of a camera.
Most of modern digital cameras have an in-built light meter, so we do not require a separate light meter. However, should we need to measure light in a methodical way, and calculate the exposure for maximum or controlled contrast, within the latitude of film (or digital image sensor), we need one light meter.
Light meter has either selenium, CdS or silicon photo-detectors. Selenium (and silicon photo-detectors) have photo-electrical sensors, which generate electricity, when light falls on it. This generated electricity is in directly proportional to the brightness of light, and it drives the pointer to show how much the light is bright.
Selenium light meter does not require external power source, hence making the device very simple, reliable and maintenance free.
However, selenium meters are not that sensitive in low light conditions and they are not very accurate. They also have "memory" which last for nearly 20 to 30 seconds.
CdS meter has photo-resistance sensor, which reads the brightness of light. It requires external power (battery) to operate. These meters are accurate and can be used in low light conditions.
Silicon photo-detector sensor has an amplifier, which calculates the brightness of the light. For this amplification, the meter requires external power source.
Most modern light meters use either CdS or silicon sensors. The meter design can be either analog or digital. Analog version shows readings with a pointer, whereas the digital design has a circuit to calculate the readings and display it on an LCD screen.
Irrespective of internal design, light meters are built to read reflected light or incident light. Some meters are equipped with a lens, and they can take readings from a narrow angle. This allows us to measure reflected light from a spot, hence the meter is referred as a spot meter.
Spot meter has an adjustment ring. Depending on the model and design, it can be adjusted from one degree to twenty degree spot.
Exposure meter has ISO setting, which has to be set before taking readings. The setting must be the same as camera's ISO setting, or same as the ISO value of the film we are using for shoot.
Read about how to measure the light and set the camera aperture and shutter speed.
Also read about:
Flash light meter
How to use a flash meter to measure light
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