An experienced photographer can take multiple readings over the shadows, midrange and highlights of the scene to determine optimal exposure, using systems like the Zone System. Taking an incident-light reading requires placing the meter at the subject's position and pointing it in the general direction of the camera, something not always achievable in practice, e.g., in landscape photography where the subject distance approaches infinity.Īnother way to avoid under- or over-exposure for subjects with unusual reflectance is to use a spot meter: a reflected-light meter that measures light in a very tight cone, typically with a one degree circular angle of view.
Because the incident-light reading is independent of the subject's reflectance, it is less likely to lead to incorrect exposures for subjects with unusual average reflectance. This pitfall (but not in the setting-sun case) is avoided by incident-light meters which measure the amount of light falling on the subject using an integrating sphere (usually, a translucent hemispherical plastic dome is used to approximate this) placed on top of the light sensor. Badly underexposed sunset photos are common exactly because of this effect: the brightness of the setting sun fools the camera's light meter and, unless the in-camera logic or the photographer take care to compensate, the picture will be grossly underexposed and dull. An unusual scene with a preponderance of light colors or specular highlights would have a higher reflectance a reflected-light meter taking a reading would incorrectly compensate for the difference in reflectance and lead to underexposure. Reflected-light meters are calibrated to show the appropriate exposure for "average" scenes. All in-camera meters are reflected-light meters. Reflected-light meters measure the light reflected by the scene to be photographed. There are two general types of light meters: reflected-light and incident-light. Photographers working with controlled lighting and cinematographers use handheld light meters to precisely measure the light falling on various parts of their subjects and use suitable lighting to produce the desired exposure levels. Many modern consumer still and video cameras include a built-in meter that measures a scene-wide light level and are able to make an approximate measure of appropriate exposure based on that. They indicate the exposure either with a needle galvanometer or on an LCD screen. Most modern light meters use silicon or CdS sensors. CdS light meters use a photoresistor sensor whose electrical resistance changes proportionately to light exposure. Silicon sensors need an amplification circuit and require a power source such as batteries to operate. Selenium sensors however cannot measure low light accurately (ordinary lightbulbs can take them close to their limits) and are altogether unable to measure very low light, such as candlelight, moonlight, starlight etc. Selenium sensors generate enough voltage for direct connection to a meter they need no battery to operate and this made them very convenient in completely mechanical cameras. Selenium and silicon light meters use sensors that are photovoltaic: they generate a voltage proportional to light exposure.
An automatic light meter/exposure unit from an 8 mm movie camera, based on a galvanometer mechanism (center) and a CdS photoresistor, in opening at left.