Pentax *ist IST DS, Operating Manual

Page: 151 / 208

149
5
F
uncti
on Re
ferenc
e
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
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*
* The value 5 used in the formula above is a fixed value
which applies only when using the built-in flash alone.
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.
Distance and Aperture when Using the Built-in
Flash
Sensitivity Built-in flash guide number
ISO200 15.6
ISO400 22
ISO800 31
ISO1600 44
ISO3200 62
Calculating Shooting Distance from Aperture Value