A properly-exposed digital sensor or film will produce an image that correctly reproduces the subject in true-to-life tones and colors. If the sensor or film is struck by too much light, the resulting image will look pale and contain less detail, and is said to be “overexposed.” If the film receives too little light, it is “under-exposed” and will look too dark.
AN EXPOSURE METER MEASURES THE BRIGHTNESS OF LIGHT
Since exposure is controlled by the camera’s shutter and aperture settings, it is essential to know what those settings should be in order to take a picture that is properly exposed. A photographer must measure the amount of light falling on or reflected by the subject to know how much of it should be allowed to strike the film. With this knowledge, the photographer can then accurately select shutter and aperture settings so that no more and no less than the proper amount of light reaches the sensor or the film.
The tool photographer’s use to know how much light is needed for correct exposure is called an “exposure meter,” or simply a “light meter.”
This versatile light meter measures both incident (or ambient) light and electronic flash.
Photographers use a number of terms to describe their use of a light meter – they say a light meter “measures, reads, takes a light reading" or "meters the light.” These terms all amount to the same thing. There is no difference in their meaning, and any one of these terms may be used throughout this site and in photography books and manuals to refer to the taking of a light meter measurement.
All exposure meters have light-sensitive cells. They convert light energy into electrical energy, which can be measured more easily. (In a sense, an exposure meter is an electrical meter containing a scale that is marked in shutter speeds and f-stops instead of volts or milliamps.)
TWO BASIC TYPES OF EXPOSURE METER
There are two basic types of light meter – the reflected light meter and the incident or ambient light meter. True to its name, the former measures the amount of light reflected from a surface (the subject's "luminance"); the ambient light meter reads the amount of light falling onto (or incident to) a subject (the illumination). There are distinct advantages to both types, and most professional photographers will have at least one of each type of meter. A third type, a multipurpose meter, is a combination of the two, and can be set to read either reflected or ambient light.
Nearly every camera sold today has a built-in exposure meter. Exposure meters that are built into cameras are reflected light meters, which means they measure the light reflected from a subject, its luminance.
Ambient light meters are separate, hand-held units – not built into a camera.
LIGHT METERS SUGGEST CAMERA SETTINGS FOR PROPER EXPOSURE
When you activate the reflected-light meter in your camera, it reads the total quantity of light reflected from the scene in the viewfinder, and provides you with combinations of shutter speeds and apertures that will usually produce proper exposure of the film in the camera.
The system is not perfect, however. For mainly dark or bright subjects, the camera's reflected-light exposure meter may tend to give a meter reading that will underexpose or overexpose the image. Reflected-light meters are calibrated for average scenes, based on the light reflected from a gray card, and photographers may have to compensate to obtain correct exposure when there are predominant areas of deep shadow or bright areas in a scene.
SOME CAMERAS SELECT EXPOSURE SETTINGS AUTOMATICALLY
Many cameras are made to automatically select a shutter speed/aperture combination for you, so that all you have to do is focus on your subject and depress the shutter. With certain cameras, you can chose to have the camera automatically select both aperture and shutter speed in the camera’s “program mode” or switch to manual mode to select the settings yourself.
(Some cameras are fully-automatic and will also do the focusing for you and even set off the flash if the light is low. These cameras are called “point-and-shoot” cameras, because in automatic mode, they do not allow the photographer to operate any of the camera’s controls, except for the shutter release button.)
Other cameras with automatic exposure systems allow you to select one variable, either the shutter speed or the aperture, and the camera will automatically adjust the other itself. You set the shutter speed when the camera is in shutter-priority mode, and select the aperture when the camera is in aperture-priority mode.
Fully-manual cameras require the photographer to select both aperture and shutter speed settings.
READ ON FOR DETAILED LIGHT METER INFORMATION
Click on the following links or on our sub-headings below, at the bottom of the page, to go to sections of the site that describe how to properly use light meters, the advantages of a reflected light meter versus an ambient light meter, and specialized meters such as those that measure the brief burst of light from electronic flash or spot meters that read small areas of a scene.
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