Pentax K-m/K2000 - Manual (Page 140)

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5
Distance and Aperture when Using
the Built-in Flash
Relationships between the guide number, aperture and distance must be
considered when shooting with the flash to obtain the correct exposure.
Calculate and adjust the shooting conditions if the flash output is not sufficient.
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*
* The value 5 used in the formula above is a fixed value
which applies only when using the built-in flash alone.
Example
When sensitivity is [ISO 100] and aperture value is F2.8
L1 = 11 ÷ 2.8 = 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 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 over-exposed.
The following equation calculates the aperture value for shooting distances.
Aperture value used F = Guide number ÷ Shooting distance
Example
When sensitivity is [ISO 100] and shooting distance is 3.5 m, aperture
value is:
F = 11 ÷ 3.5 = 3.1
If the resulting number (3.1, 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.
ISO Sensitivity Built-in flash guide number
ISO 100 Approx. 11
ISO 200 Approx. 15.6
ISO 400 Approx. 22
ISO 800 Approx. 31.1
ISO 1600 Approx. 44
ISO 3200 Approx. 62.2
Calculating Shooting Distance from Aperture Value
Calculating Aperture Value from Shooting Distance