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Working diagram of the exposure meter The diagram shows the relationship between film speed Sv (speed value) and brightness Bv (brightness value) on the one hand, and between shutter speed Tv (time value) and aperture Av (aperture value) on the other, together with the resulting exposure value Ev. The diagram is in two parts connected by diagonal lines that represent the exposure values Ev. A typical example, marked in red, shows how these values correlate. Assuming a film speed of ISO 400/27°, follow the vertical line to the point where it intersects with the horizontal line for brightness, in this case 0.5 cd/m2, typical for
A diagonal which passes through this point of intersection leads to the relevant exposure value, in this case Ev 4. Various combinations of aperture and shutter speed can produce this value, i.e. transfer to the camera's working range.
For correct exposure, the points of intersection of the vertical Av and the horizontal Tv lines must always lie exactly on a diagonal Ev line. In the example, three such combinations are shown: A = stop 2 at ¼s; B = stop 8 at 4s; and C = stop 16 at 15s. Each corresponds to the correct exposure value.
In the modes shutter priority and aperture priority, one of these values is preset, the corresponding value is set automatically; in automatic program mode, the camera automatically sets both.
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