Pentax K20D User Manual
Page 142
141
Usi
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Fl
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5
Distance and Aperture when Using
the Built-in Flash
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 flash output is not sufficient.
Built-in flash guide number
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 = 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, 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 m, aperture value is:
F = 13 ÷ 3 = 4.3
If the resulting number (4.3, in the above example) is not available as
a lens aperture, the smaller number that is closest (4.0, in the above
example) is generally used.
ISO Sensitivity
Built-in flash guide number
ISO 100
13
ISO 200
18.4
ISO 400
26
ISO 800
36.8
ISO 1600
52
ISO 3200
73.5
Calculating Shooting Distance from Aperture Value
Calculating Aperture Value from Shooting Distance