STEREO PROCESSING 305inore rapidly than the shadows. Thus, the normal-negative pro-
gresses in both contrast and density as development proceeds. It
is also known that the density grows more rapidly after an in-
creased exposure. Thus it follows that if the exposure is increased,
printable density will be reached coincident with a lower degree
of contrast than would otherwise be the case. Thus, by increasing
the exposure and decreasing the development, it is possible to
decrease the overall contrast of the negative. This is the essence
of control development, and stereo development is above all
control development.
The possibilities of control development are not fully under-
stood by the average amateur. Consider an emulsion normally
rated at ASA 25. By the use of a high-energy developer, a print-
able negative may be obtained with 1/4 normal exposure. This in
turn means an exposure based upon a sensitivity of ASA 100. But
if a fine-grain, superficial developer is used, four times normal
exposure is necessary, or an effective sensitivity of ASA 6.25. Yet
only one film is under consideration, and while it does have a
fixed primary sensitivity to light, its sensitivity in relation to the
developed image actually ranges from ASA 6+ to ASA 100, or a
range equivalent to that from a slow positive emulsion to a high-
speed panchromatic!
Degree of Deviation.-The exposure is that required to record
the deepest shadow detail of significant size... and that means
anything larger than a pinhead in the negative. The deep shadow
is read by holding the meter close to it. The highlight is read at
its brightest exclusive of the rest of the scene. The normal photo-
graphic range from shadow to highlight is 1:ioo (the ”normal”
negative has less than gamma 1.0, so such a recorded range is
developed to about i:60 to i:80 and can be printed on paper).
Suppose your shadow reads four and highlight 800. The range is
1:200. You multiply the minimum reading by 50, the “latitude”
factor, 4 x 50=200 and give an exposure indicated for zoo maxi-
mum highlight (meter = 50) regardless of the^800 highlight. This
is only a rough rule but is usually near enough. But with^200 as
the maximum, highlights over 400 will be blocked.
It requires a bit of experience to learn the exposure control,
but the exposure indicated for a maximum highlight of 50 times
the deepest shadow usually works out.