Pentax K-7 Operating Manual page 55

A set criteria is necessary between the guide number, aperture and
distance when shooting with the flash to obtain the correct exposure.
Calculate and adjust the shooting conditions if the flash output is not
sufficient.
ISO sensitivity Built-in flash guide number
ISO 100 Approx. 13
ISO 200 Approx. 18.4
ISO 400 Approx. 26
ISO 800 Approx. 36.8
ISO 1600 Approx. 52
ISO 3200 Approx. 73.5
The following equation calculates the distance of the flash for aperture values.
Maximum flash distance L1 = Guide number + Aperture value
Minimum flash distance L2 = Maximum flash distance + 5*
*The value 5 used in the formula above is a fixed value
which applies only when using the built-in flash alone.
Example: When the sensitivity is [ISO 100] and aperture value is F2.8
L1 = 13 + 2.8 = approx. 4.6 (m)
L2 = 4.6 + 5 = approx. 0.9 (m)
Therefore, the flash can be used in a range of about 0.9 m to 4.6 m.
The flash cannot be used when the distance is 0.7 m or less. When the
flash is used at closer than 0.7 m, it causes vignetting in the picture corners,
the light is distributed unevenly and the picture may be overexposed.
The following equation calculates the aperture value for shooting
distances.
Aperture value F = Guide number + Shooting distance
Example: When the sensitivity is [ISO 100] and shooting distance is 4 m,
aperture value is
F= 13+4=3.25
If the resulting number (3.25, 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.