104
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
fl
ash. Calculate and adjust the shooting conditions if
fl
ash is not suf
fi
cient.
●
Built-in
fl
ash guide number
Sensitivity
Built-in
fl
ash guide number
ISO 100
13
ISO 200
18.4
ISO 400
26
ISO 800
36.7
ISO 1600
52
Calculating Aperture Value from Shooting Distance
The following equation calculates the aperture value for shooting distances.
Aperture Value Used F = Guide number ÷ Shooting distance
Example)
When sensitivity is [ISO 200] and shooting distance is 6.1 m, aperture value is:
F = 18.4 ÷ 6.1 = 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.
Using the Built-in Flash
Calculating Shooting Distance from Aperture Value
■
The following equation calculates the distance of the
fl
ash for aperture
values.
Maximum
fl
ash distance L1 = Guide number ÷ Selected aperture
Minimum
fl
ash distance L2 = Maximum
fl
ash distance L1 ÷ 5*
* The value 5 used in the formula above is
a
fi
xed value which applies only when
using the built-in
fl
ash alone.
Example)
When sensitivity is [ISO 200] and aperture value is F4
L1 = 18.4 ÷ 4 = approx. 4.6 (m)
L2 = 4.6 ÷ 5 = approx. 0.9 (m)
Therefore, the
fl
ash can be used in a range of about 0.9 m to 4.6 m. The
fl
ash cannot
be used when the distance is less than 0.8 m. When the
fl
ash is used at closer than
0.8 m, it causes vignetting in the picture corners, light is distributed unevenly and the
picture may be over-exposed.
Summary of Contents for GX-20 - Digital Camera SLR
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