FLASH IN STEREO 117Thus you can have the flash intensity, at the principal object,
below daylight, equal to it or greater than it is.
You always have a certain daylight level upon the principal
subject, so the flash is never the sole illumination. The subject
illumination, therefore, is the daylight plus the flash. Because
the subject is often lighted at a lower level than the general
scene, that is when there are cast shadows, the flash usually (but
not always) is designed to raise the level to that point where it
will equal or fall slightly short of the general daylight level.
The principal object will have an appearance which is deter-
mined by flash plus daylight, while the general scene will have
the appearance due to daylight alone. The two lights are self-
blending as to intensity, and no sharp line of demarcation will
be seen, regardless of the flash intensity.
(A) Flash below daylight level. This is the normal procedure
when the normal guide number is used to select the flash ex-
posure. (B) Flash equal to daylight. This tends to eliminate cast
shadows completely, and to substitute flash modeling for natural
modeling. (C) Flash brighter than daylight. This is used for
stopping motion and also for pictorial effects because it causes
the general scene to be darkened to a degree depending upon the
relative intensities of the two lights, providing the principal sub-
ject is correctly exposed.
In all of these conditions it is assumed that there is some prin-
cipal object, a figure or a group of comparatively limited size.
Flash will not ordinarily be satisfactory if the distance of such
object from the camera varies through a distance equal to more
than one-third the distance of the nearest part of the subject from
the camera. For example, if a group of people is so placed that the
farthest member is three feet away from the nearest, then the flash
should be at least nine feet from the nearest member.
The relative intensity of lights upon the nearest member will
be g X g or 81 units, that of the member three feet farther will be
12 X 12 or 144. Taken in inverse order the light intensities are
on the order of 1/81 to 1/144, so the farthest object receives a
little more than half the illumination of the nearest.
As we have seen, the number 25 bulb will approximately aver-
age daylight intensity at ten feet. This of necessity is a very vague
statement, as “average” daylight may be twice as bright as the