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Ap
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8
The flash cannot be used when the distance is less than 0.7m. When the
flash is used within 0.7m, it causes vignetting in the picture corners, light
is distributed unevenly and the picture may be over-exposed.
* The value 5 used in the formula above is a fixed value which applies to
using the built-in flash alone.
The guide number depends upon the sensitivity (ISO) used as shown
below.
This example uses ISO200 with an aperture of f/4.
Guide number (15.6) ÷ f/4 = 3.9m
3.9 ÷ 5 0.8m
Therefore, the flash can be used in a range of about 0.8m to 3.9m.
If the resulting number (3) is not available as a lens aperture, the smaller
number (2.8) is generally used.
Calculating flash distance from the aperture used
(using built-in flash)
Maximum flash distance= Guide Number ÷ Selected aperture
Minimum flash distance= Maximum flash distance ÷ 5* (Note)
ISO200
→
15.6
ISO800
→
31
ISO3200
→
62
ISO400
→
22
ISO1600
→
44
Calculating aperture used from the flash distance
(using the built-in flash)
Guide number ÷ camera-to-subject distance = Aperture
