Pentax K100 D Operating Manual page 159

Calculating Shooting Distance from Aperture Value
The following equation calculates the distance of the flash for aperture
values.
Maximum flash distance L1 = Guide number ÷ Selected aperture
Minimum flash distance L2 = Maximum flash distance ÷ 5*
Example
When sensitivity is [ISO 200] and aperture value is F4
L1 = 15.6 ÷ 4 = approx. 3.9 (m)
L2 = 3.9 ÷ 5 = approx. 0.8 (m)
Therefore, the flash can be used in a range of about 0.8 m to 3.9 m.
The flash cannot be used when the distance is less than 0.7 m. When
the flash is used at closer than 0.7 m, it causes vignetting in the picture
corners, light is distributed unevenly and the picture may be over-
exposed.
Calculating Aperture Value from Shooting Distance
The following equation calculates the aperture value for shooting
distances.
Aperture value used F = Guide number ÷ Shooting distance
When sensitivity is [ISO 200] and shooting distance is 5.2 m, aperture
value is:
F = 15.6 ÷ 5.2 = 3
If the resulting number (3, in the above example) is not available as a
lens aperture, the smaller number that is closest (2.8, in the above
example) is generally used.
* The value 5 used in the formula above is a fixed value
which applies only when using the built-in flash alone.
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