Illumination of an object is falling at a luminous flux as measured by contrast, requires a wide angle of coverage (up to 180 °). This is achieved by setting the diffusely scattering plate (glass of milk) in front of the photocell window.
While working on portraits in artificial light, when the frame visible lights, table lamps and chandeliers, the meter readings will also be overvalued and shadow areas of the frame will ill-conditioned.
Consider two examples from practice:
1. Brightly lit figure of a man is a relatively small portion of the frame and located on a dark, low-lit background.
2. The same figure is brightly lit with a light background.
Indications meter built into the device, at a total
Measuring the brightness in the first and second case, despite the same light the main object will be different. In the first case
Exposure levels in relation to the human figure will be inflated. This will lead to the fact that an important part of the plot will be overexposed negatives. In the second case, the magnitude of exposure, the exposure meter was found to be correct, because the difference between the brightness of the object and background is small.
This method of measurement is useful when shooting portraits and close-up, because you can not only determine the overall exposure, but also to measure the most illuminated part of the face, and measuring the brightness of the shaded part of the control the ratio of black and white drawing of a portrait of brightness. This version of the definition of exposure is to measure the brightness of the subject areas.
There is another method of determining exposure - by measuring the light. In this case, the meter is installed in close proximity to the subject matter and toward the main light and a photocell put milk glass.