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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)

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.

ISO20015.6 ISO80031 ISO320062

ISO40022 ISO160044

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.

Calculating aperture used from the flash distance (using the built-in flash)

Guide number ÷ camera-to-subject distance = Aperture

If the resulting number (3) is not available as a lens aperture, the smaller number (2.8) is generally used.

8 Appendix