The interactive calibration method was inspired by photography, but
our intention is not to apply exact photographic methods. We will
adopt some terminology from photography and use it in a simplified
way. In this paper the aperture value is defined as 0, if and only if,
the average luminance value, 
, is in the center of the
clipping window on the linear scale. When displayed on the log scale
it is in the upper half of the interval. Setting the aperture to +1
means shifting the value by 1 on the 
scale, which
doubles it. In our implementation the aperture can be set to 
, 
, 
,...,
. Of course, the aperture can be
set to larger values or a finer step can be applied, yet, from the
hundred years of photographic experience and from our experience, this
is almost never necessary.