Flash Effective Range; Calculating The Flash Effective Range - Pentax AF-540FGZ Operating Manual

Flash Effective Range

Calculating the Flash Effective
Range
When setting the lens to manual f/stop, calculate the
guide number at full flash with the use of flash's zoom
position and ISO. Divide the resulting guide number by
the aperture in use. Thus, the maximum distance is
obtained. The minimum distance is obtained in dividing
this max. distance by approx. 10.
Example:
With a 85 mm lens at f/4 and sensitivity at ISO100
 For the zoom position 85mm and sensitivity at ISO100, the guide number is 54.
 54 (guide number) /4 (aperture) = 13.5m (max. distance)
 13.5m (max. distance) /10 = 1.35m (min. distance) Thus, flash effective range is approx. 1.35m - 13.5m.
80
However, if the minimum distance obtained is 0.7m or
less, the minimum distance will be 0.7m. Refer to page
82 for the guide number table.