Pentax ist ist DL User Manual

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Function Reference
The following equation calculates the aperture value for shooting
distances.
Aperture Value Used F = Guide number ÷ shooting distance
When sensitivity is [ISO200] and shooting distance is 5.2 m, aperture
value is:
F = 15.6 ÷ 5.2 = 3
If the resulting number (3, 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.
Calculating Aperture Value from Shooting Distance

Pentax ist ist DL Specifications

General

Brand	Pentax
Model	ist ist DL
Category	Digital Camera
Language	English