illumination from the centre to the edges of the camera
format, which of course increases the effective brightness
range.
2.7• Construction of Tables for use with the Maximum-Minimum
Brightness Meter
The method of constructing the tables referred to in
paragraph 2.5* has been described by the writer in another
paper 0). The principle is to dispose the maximum and
minimum subject brightnesses at two negative density
levels in such a way as to secure the highest possible
resolution over the whole brightness range. Consider
figure 1. Let M. and N. be two density levels which for
a particular film confine resolving power to certain
acceptable limits and let A. and B. be maximum and minimum
subject brightnesses. The object is to spread the
subject brightness range exactly between these limits,
such that the development gamma is The measured
subject brightnesses A and B are shown plotted on the
exposure scale with respect to the light flux that would
enter the camera when the exposure is l/300th second at
P.5.6. This i3 called the "standard exposure" and is
used as a basis from which to deduce particular exposure.
B
—I 2.8. Shown also in figure 1 is a selection of character
istic curves for various emulsions developed to produce
_ different gammas. The different positions of these
curves on the log exposure axis is indicative of their
various speeds. A curve must now be chosen, having a
slope or gamma equal to Lid and the brightness range AB
must be shifted exactly into position under this curve.
The amount of this shift is equal to the difference between
the "standard exposure", and the exposure to give the
desired negative densities. Each 0.3 shift is equivalent
to a change of exposure of one stop. The exposure
required is determined in this way but with two further
criterions
1 • The change of exposure indicated by the shift
must be possible for the camera being used
without decreasing the shutter speed beyond the
limits necessary to avoid significant image
movement.
2. Where there are various alternative possibilities,
one chooses the emulsion having the finest grain.
Tables constructed on this basis would enable a photographer
to read off opposite the entry of his maximum/minimum meter
readings, the best film to use, the exposure to give to it,
and the development time to match that exposure. In
practice it would be found unnecessary to consider more
than two different classes of air film.