As aforementioned in the article on light intensity and dynamic range, every photo taken with a digital camera is a combination of the factors shutter speed, aperture and ISO. In full manual mode of a camera, a photographer always has to adjust each of these factors individually which requires a lot of experience. In semi-automatic or full-automatic mode of a camera, these settings are adjusted by the camera.
While many experienced photographers prefer to use full manual mode, they either have to estimate the illuminance and adjust their camera accordingly, or they will have to measure the illuminance. There are two very different approaches to measure the illuminance, incident light metering and reflected light metering, as described below:
Incident light metering. This type of measurement is done by using a handheld light meter, a specialized device that obtains information on the intensity of light falling on the subject. Therefore, the light meter is held in close proximity to the subject, pointing to the main source of light. Depending on the surface and color of the subject, the photographer can assess the light being reflected from the subject towards the camera which in turn gives an indicative value for the settings to be used. This method is considered to be very accurate as it only includes the light situation at the subject and ignores light situations at different objects. This will allow to find the perfect exposure for the desired subject, but as different objects in the scene remain unconsidered these are likely to be over- or underexposed in the final picture. Also, incident light metering is only an option for photography in studios or comparable setups where the subject is easily accessible. For any other scenes such as landscape and architecture where the photo does not consist of a small subject but rather a large scene, incident light metering is impossible to realize.
Reflected light metering. This type of light metering measures the amount of light that is reflected by a particular scene or subject. For this reason, the metering device must be faced towards the scene from the position the camera is located. While reflected light metering is also available as a feature on many handheld light meters, todays cameras mainly use built-in light meters as described in the article on viewfinder optics.
This metering technique takes a reading of the entire scene (highlight and shadow areas) and provides an averaged reading designed to record the scene or subject as a middle gray. This gray is a standard value designed to provide a relatively safe exposure for average subjects.
Reflected light metering is a very quick and profitable method as neither information on the original source of light nor the material of the subject are required. Also, in case that photographic filters are applied to lenses which do always absorb a small portion of the incident light, this absorption is already considered by reflected light metering. However, as aforementioned, the most convincing argument for reflected light metering is that most situations do not allow incident light metering because the scene is too far away from the photographer or the light situation is changing rapidly as for example at concerts or sport events.
Unfortunately, one noticeable drawback of reflected light metering is that it can change the natural appearance of subjects in case these are not average in color. For example, if the camera has to evaluate the exposure of a bride in a white dress, the built-in light meter will perceive a lot of light being reflected by the dress and recommend a lower exposure than is needed in order to balance the picture to middle gray. As amresult, the image will be underexposed and the dress will look gray. Conversely, simply relying on the camera to meter a person in a black suit, the light meter would see very little light being reflected by the suit and recommend a higher exposure than is needed in order to balance it to middle gray. As a result, the image will be overexposed and the suit will look gray again. These problems are the result
of measuring reflected light from the subject rather than the actual light falling on the subject.
To avoid this type of overcompensation in extreme situations (snow, white dress etc.) it is important to know the metering principle and go for manual exposure compensation. This means that it can be advisable to tell the camera to deliberately overexpose a snow landscape or wedding dress so that white details will be truly white in the picture. It must be noted though that overexposing white scenes is risky and may end up in burnt highlights. A safer method would be to trust the camera – knowing that it may slightly overcompensate – and brighten up white details later.
Middle gray. example with black and white wall.