Measuring Reflected and Incident Light of a Digital Camera

Until now we have supposed that the measuring of light of a zone is made by framing it completely in the viewfinder; meaning, we measure the light reflected by that zone. Now we know that the intensity of the reflected light depends not only on the intensity of incident light and the orientation of the surface in regard to the source, but also the reflectance of the surface in question. Nonetheless, the photometer of a camera doesn't take this factor in account, since it considers that the intensity measured refers to a medium reflectance surface (18%). For this reason, in the case that a surface has a very high or very low reflectance, the measurement will be incorrect . Even with a homogenous illumination, to surfaces of very different reflectance can cause noticeable contrast.

If for example, the background of an image is snow or a very bright surface, the photometer will be wrong in it's indications because it would be considering that the measured intensity (too high), corresponds to a normal reflectance surface; as consequence, the exposure value recommended for this zone will be higher that what it really indicates (underexposure is caused); in the image, all the tones will appear much darker than in the scene, except the shadows. On the other hand, is an extensive zone of the frame is of very low

reflectance, the photometer may recommend a very low value, with overexposure and risk for the lights. In those cases, it's necessary to be sure that the measurement is made on a medium reflectance surface, such as the palm of the hand or an 18% gray piece of cardboard (used in studios for precise measurements). In this way the zones of medium reflectance will stay correctly exposed and those of very high or very low reflectance occupy the place they correspond in the exposure latitude.

Measuring Incident Light of a Digital Camera

The measuring of reflected light requires frequent corrections when the zone of the frame presents a very high or very low reflectance. On the other hand, a measurement of incident light, situated on the point of the scene who's brightness we want to measure, captures and values the light which falls on that point in a (solid) 180º angle. The measurement is made on a semispherical dome of translucent material, whose pole must be directed towards the camera. This way, the frontal light (from the same direction as the camera) has predominance over lateral lights. The measurement of incident light lets us compare in a more reliable way the different brightness levels of the frame, although it has the inconvenience that it's not always possible to place the photometer on the point of the scene who's brightness we wish to measure (exteriors, landscapes).