Pentax Silkypix K-500, Operating Manual

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Relationships among the guide number, aperture and distance must be
considered when shooting with the flash to obtain a correct exposure.
Calculate the shooting distance or aperture value in the following way,
and adjust the shooting conditions if the flash output is not sufficient.
The following equation calculates the effective distance of the flash.
Maximum flash distance L1 = Guide number ÷ Aperture value
Minimum flash distance L2 = Maximum flash distance ÷ 5 *
* The value used in the formula above is a fixed value
which applies only when using the built-in flash alone.
Example:
When the sensitivity is ISO 200 and the aperture value is F5.6:
L1 = 17 ÷ 5.6 = approx. 3 (m)
L2 = 3 ÷ 5 = approx. 0.6 (m)
Therefore, the flash can be used in a range of about 0.6 m to 3 m.
However, the built-in flash in this camera cannot be used when the
distance is 0.7 m or less. When the flash is used at distances closer
than 0.7 m, it causes vignetting in the picture corners, light is
distributed unevenly and the picture may be overexposed.
Distance and Aperture when Using the Built-in
Flash
ISO Sensitivity
Built-in Flash
Guide Number
ISO Sensitivity
Built-in Flash
Guide Number
ISO 100
Approx. 12
ISO 3200
Approx. 68
ISO 200
Approx. 17
ISO 6400
Approx. 96
ISO 400
Approx. 24
ISO 12800
Approx. 136
ISO 800
Approx. 34
ISO 25600
Approx. 192
ISO 1600
Approx. 48
ISO 51200
Approx. 272
Calculating Shooting Distance from Aperture Value
K-500.book Page 252 Friday, June 14, 2013 2:29 PM