Distance And Aperture When Using The Built-In Flash - Pentax ist ist DL Operating Manual

Distance and Aperture when Using the Built-in
Flash
A set criteria is necessary between the guide number, aperture and
distance when shooting with the flash.
Calculate and adjust the shooting conditions if flash is not sufficient.
Flash guide number for built-in flash
Sensitivity Built-in flash guide number
ISO200
ISO400
ISO800
ISO1600
ISO3200
Calculating Shooting Distance from Aperture Value
The following equation calculates the distance of the flash for aperture
values.
Maximum flash distance L1 = Guide Number ÷ Selected aperture
Minimum flash distance L2 = Maximum flash distance ÷ 5*
Example
When sensitivity is [ISO200] and aperture value is F4
L1 = 15.6 ÷ 4 = approx. 3.9 (m)
L2 = 3.9 ÷ 5 = approx. 0.8 (m)
Therefore, the flash can be used in a range of about 0.8 m to 3.9 m.
The flash cannot be used when the distance is less than 0.7 m. When
the flash is used at closer than 0.7 m, it causes vignetting in the picture
corners, light is distributed unevenly and the picture may be over-
exposed.
15.6
22
31
44
62
* The value 5 used in the formula above is a fixed value
which applies only when using the built-in flash alone.
149
5