Pentax K-r Operating Manual page 39

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Relationships between the guide number, aperture and distance must be
considered when shooting with the flash to obtain a correct exposure.
Calculate and adjust the shooting conditions if the flash out 3ut is not sufficient.
Built-in Flash
Sensitivity Guide Number
ISO 100 "1 Approx. 12
ISO 200 Approx. 16
ISO 400 Approx. 24
ISO 800 Approx. 32
ISO 1600 Approx. 48
I Built-in Flash
Sensitivity. Guide Number
ISO 3200 Approx. 64
ISO 6400 Approx. 96
ISO 12800 "1 Approx. 128
ISO 25600 "1 Approx. 192
"1 This can be used when [3. ExpandedSensitivity]in the [C CustomSetting 1]menu is set
to [On].
The following equation calculates the distance of the flash for aperture values.
Maximum flash distance L1 = Guide number + Aperture value
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 the sensitivity is ISO 200 and the aperture value is F4.0
L1 = 16 + 4.0 = approx. 4 (m)
L2 = 4 + 5 = approx. 0.8 (m)
Therefore, the flash can be used in a range of about 0.8 m to 4 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 closer than 0.7 m, it causes
vignetting in the picture corners, light is distributed unevenly and a picture
may be overexposed.
The following equation calculates the aperture value for shooting distances.
Aperture value F = Guide number + Shooting distance
Example)
When the sensitivity is ISO 200 and the shooting distance is 5 m, the
aperture value is:
F=16+5=3.2
If the resulting number (3.2, 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.