Pentax K-X User Manual

Page 162

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5

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 the correct exposure.

Calculate and adjust the shooting conditions if the flash output is not sufficient.

The following equation calculates the distance of the flash for aperture values.
Maximum flash distance L1 = Guide number ÷ Aperture value
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 the sensitivity is ISO 200 and the aperture value is F4.0
L1 = 16 ÷ 4.0 = approx. 4 (m)

L2 = 4 ÷ 5 = approx. 0.8 (m)
Therefore, the flash can be used in a range of about 0.8 m to 4 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, the light is distributed unevenly and the

picture may be overexposed.

The following equation calculates the aperture value for shooting distances.
Aperture value F = Guide number ÷ Shooting distance
Example)

When the sensitivity is ISO 200 and the shooting distance is 5 m, the

aperture value is:
F = 16 ÷ 5 = 3.2
If the resulting number (3.2, 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.

ISO Sensitivity

Built-in Flash Guide Number

ISO 200

Approx. 16

ISO 400

Approx. 24

ISO 800

Approx. 32

ISO 1600

Approx. 48

ISO 3200

Approx. 64

ISO 6400

Approx. 96

Calculating Shooting Distance from Aperture Value

Calculating Aperture Value from Shooting Distance

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