Pentax ist - Manual (Page 115)

Example:
If an ISO100 film is used at an aperture of f/3.5, the flash
effective distance is obtained as follows:
For maximum distance Guide Number (11) ÷ f/3.5 = 3.14m
For minimum distance
3.14 ÷ 5 = 0.63m
→0.7m *
* Minimum distance of the built-in flash
Thus, the flash effective distance is from approx. 0.7m to 3.14m.
2. Calculating the aperture according to the camera-to-subject
distance
Aperture = Guide Number ÷ Camera-to-subject distance
Example:
If the camera-to-subject distance is 2.5m and ISO 100 film is
used, the effective aperture is obtained as follows:
Guide Number (11) ÷ Distance (2.5m) = f/4.4
If the calculated aperture value is different from an indicated f
stop, for instance f/4.4, select the next smaller aperture value.
(Set at f/4 in this case.)
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