Pentax K-X Operating Manual page 50

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Relationships between the guide number, aperture and distance must be
considered when shooting with the flash to obtain the correct exposure.
Calculate and ac
ISO Sensitivity
ISO 200
ISO 400
ISO 800
ISO 1600
ISO 3200
ISO 6400
ust the shooting conditions if the flash output is not sufficient,
Built-in Flash Guide Number
Approx. 16
Approx. 24
Approx. 32
Approx. 48
Approx. 64
Approx. 96
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 200 and the aperture value is F4.0
L1 = 16 + 4.0 = approx. 4 (m)
L2 = 4 + 5 = approx. 0.8 (m)
Therefore, the flash can be used in a range of about 0.8 m to 4 m.
However, the built-in flash in this camera 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 200 and the shooting distance is 5 m, the
aperture value is:
F=16+5=3.2
If the resulting number (3.2, 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.