Pentax AF 540 FGZ II Bedienungsanleitung

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Calculating the Flash Effective Range

When setting the lens to manual F/stop, calculate the guide 

number at full flash with the use of flash’s zoom position and 

ISO. Divide the resulting guide number by the aperture in use. 

Thus, the maximum distance is obtained. The minimum 

distance is obtained in dividing this max. distance by approx. 

10. However, if the minimum distance obtained is 0.7 m or less, 

the minimum distance will be 0.7 m. Refer to page 58 for the 

guide number table.

3. Appendix

Flash Effective Range

Example:

With an 85 mm lens at F/4 and sensitivity at ISO100

1

For the zoom position 58mm and sensitivity at ISO100, 

the guide number is 54.

2

54 (guide number) /4 (aperture) = 13.5m (max. distance)

3

13.5m (max. distance) /10 = 1.35m (min. distance) Thus, 

flash effective range is approx. 1.35m - 13.5m.

* The numbers used in this example are for K series (APS-C).

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