You Do Not Have to Be a Pro to Set Correct Exposure
Exposure in photography is the total amount of light allowed to fall on the photographic medium - photographic film or image sensor - during the process of taking a photograph. The correct exposure means that enough light reaches the photographic medium – if there is too little light, the image will be underexposed, if there is too much light, it will look overexposed. That is why exposure is considered one of the most important settings affecting the quality of the final photograph.
Exposure is measured in exposure value (EV), with higher values denoting more light. Exposure value indicates an interval on the photographic exposure scale, with 1 EV corresponding to a standard power-of-2 exposure step, commonly referred to as a “stop’’. When we change an exposure setting to 1 EV, it corresponds to the change in aperture on +/- 1 division, and accordingly increasing or reducing a shutter speed twice.
For example, if your camera set an exposure of 1/125 and f/8 and you want to change this settings to +1 EV, the camera may set to any of these pairs: 1/60 and f/8; 1/90 and f/6.7; 1/125 and f/5.6. In any of these cases twice as much light will reach a sensor.
When we shoot objects with a small range of brightness, it is not difficult to set a correct exposure. The problem appears when we want to include in a photograph two objects, which are not equally lit, for example a dark forest and bright sky.
In this case either a forest will look too dark, or the sky will be washed out. But sometimes we may use this disadvantage to add special effects to our images. For example, we can get a neat silhouette image when photographing a man with the light behind him.
If you don’t have a certain experience, it is difficult to determine proper exposure, as our eye can not always measure the intensity of light correctly. That is why there are special devices for metering illumination, which are called light meters.
The meter will show the camera settings (f-number and shutter speed) that nominally result in the “best” picture. A light meter is also used in illumination, whether as a part of a larger light regulation system or just as a handheld device used to check whether light levels meet specified criteria. A handheld light meter used in illumination usually indicates levels of illuminance or luminance rather than showing camera settings.
1. Several types of light meter have been used, the three most common incorporating selenium, CdS, and silicon photo sensors. Read more here.
2. 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. Professional photographers generally use handheld ambient-light meters to precisely measure the light falling on various parts of their subjects, and use suitable lighting to produce the desired exposure levels.
3. There are two general types of ambient-light meters: reflected-light and incident-light.
* Reflected-light meters measure the light reflected by the scene to be photographed. All in-camera meters are reflected-light meters. Reflected-light meters are calibrated to show the appropriate exposure for “average” scenes. 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.
* This pitfall 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). As the incident-light reading is independent of the subject’s reflectance, it cannot lead to incorrect exposures for subjects with unusual average reflectance.
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 is at infinity. In situations where very precise and consistent exposure is required (e.g., cinematography, where errors and variations in exposure are unacceptable and can be very costly), incident-light meters are preferred.
* Another 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 angle. 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. Many modern cameras include sophisticated multi-segment metering systems that measure the luminance of different parts of the scene and use various algorithms to determine optimal exposure.
4. There are other types of specialized photographic light meters. Flash meters are used in flash photography to verify correct exposure. Color meters are used where high fidelity in color reproduction is required. Densitometers are used in photographic reproduction.
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