Pentax ZX-5N - Manual (Page 77)

Calculating the flash effective distance according to
the camera-to-subject distance.
Guide Number (11) f/2.8
3.9 -i" 5 = 0.8m
3.9m
Maximum flash distance = Guide Number Selected
aperture
Minimum flash distance = Maximum flash distance h -
5*
When the distance to the subject is less than 0.7m
(2.3ft), the flash cannot be used. If the flash is used
within that distance, it causes vignetting in the picture
corners, light is distributed unevenly and the picture
may be overexposed.
• The value 5 used in the formula above was obtained
from the built-in flash.
The guide number (GN) depends upon the film speed
used as shown below.
IS025 GN5.5 IS0200 ^ GN15.6
IS050 GN7.8 IS0400 ^ GN22
IS0100 ^ GN11
Thus, the flash effective distance is from approx. 0.8m
to 3.9m.
Calculating the aperture according to the camera-to-
subject distance
Aperture = Guide Number Camera-to-subject dis­
tance
If the calculated aperture value is different than an indi­
cated f-stop on the aperture ring, for instance f/3,
choose the next smallest aperture ring number (f2.8 in
this case).
Calculating the camera-to-subject distance in the
Shutter-Priority AE Mode.
The camera-to-subject distance can be calculated using
the above mentioned formula. However, in the Shutter-
Priority AE Mode, the-camera-to-subject distance will
change depending on what aperture is set.
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If an IS0100 film is used at an aperture of f/2.8, the
flash effective distance is obtained as follows: