The method of mean light intensity is the most commonly used method
in non-professional photography since in most cases it gives
acceptable results. According to [Morv84] the method gave good results
in 80% of amateur motifs (the sample was ten thousand motifs). In
the remaining 20% the aperture should be shifted up or down to
obtain optimal results. E.g., if the main subject is in shadow,
an average measure causes the main subject to appear only as a
silhouette. For the sake of a display rich in detail, the aperture
often has to be opened by 2 or 3 units, compared to the measured
mean value. This means that the desired result would be obtained from
a mean illumination twice or three times higher on the 
scale - i.e. the value 4 or 8 times higher than the average value is
displayed as "medium gray". Thus, the clipping window has to be
shifted on the scale up to 3 units compared to values
resulting from the average measurement method. For strong contrast
subjects in professional photography, a spot metering is made for
all the typical details, that may result in a subject contrast of
even several hundreds or more. Then, a contrast interval of 32 has to
be selected, which produces the least loss due to the forced
clipping. The fundamental method presented in this work follows
that idea.