Pentax K20D manual Distance and Aperture when Using, Built-in Flash

Distance and Aperture when Using	141
the Built-in Flash

A set criteria is necessary between the guide number, aperture and distance when shooting with the flash to obtain the correct exposure. Calculate and adjust the shooting conditions if flash output is not sufficient.

Built-in flash guide number

ISO Sensitivity	Built-in flash guide number
ISO 100	13
ISO 200	18.4
ISO 400	26
ISO 800	36.8
ISO 1600	52
ISO 3200	73.5

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*

*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 [ISO 100] and aperture value is F2.8 L1 = 13 ÷ 2.8 = approx. 4.6 (m)

L2 = 4.6 ÷ 5 = approx. 0.9 (m)

Therefore, the flash can be used in a range of about 0.9 m to 4.6 m. The flash 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, light is distributed unevenly and the picture may be over-exposed.

Calculating Aperture Value from Shooting Distance

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

Example

When sensitivity is [ISO 100] and shooting distance is 3 m, aperture value is: F = 13 ÷ 3 = 4.3

If the resulting number (4.3, in the above example) is not available as a lens aperture, the smaller number that is closest (4.0, in the above example) is generally used.

5

Using the Flash