Distance And Aperture When Using The Built-In Flash - Pentax K-5II Operating Manual

Distance and Aperture when Using the Built-in Flash

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 output is not sufficient.
Sensitivity Built-in Flash Guide Number
ISO 100
ISO 200
ISO 400
ISO 800
ISO 1600
*1 This can be used when [3. Expanded Sensitivity] in the [A Custom Setting 1] menu is set to [On].
Calculating Shooting Distance from Aperture Value
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 *
Example: When the sensitivity is ISO 100 and the aperture value is F2.8
L1 = 13 ÷ 2.8 = approx. 4.6 (m)
L2 = 4.6 ÷ 5 = approx. 0.9 (m)
Therefore, the flash can be used in a range of about 0.9 m to 4.6 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.
Calculating Aperture Value from Shooting Distance
The following equation calculates the aperture value for shooting distances.
Aperture value F = Guide number ÷ Shooting distance
Example: When the sensitivity is ISO 100 and shooting distance is 4 m,
the aperture value is:
F = 13 ÷ 4 = 3.25
If the resulting number (3.25, 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.
Approx. 13
Approx. 18.4
Approx. 26
Approx. 36.8
Approx. 52
* The value 5 used in the formula above is a fixed value
which applies only when using the built-in flash alone.
Sensitivity Built-in flash guide number
ISO 3200
ISO 6400
ISO 12800
ISO 25600 *1
ISO 51200 *1
Approx. 73.5
Approx. 104
Approx. 147
Approx. 208
Approx. 294
165
5