Pentax K-50 specifications 244, Calculating Shooting Distance from Aperture Value

244

9

Using the

Flash

Distance and Aperture when Using the Built-in Flash

Relationships among the guide number, aperture and distance must be considered when shooting with the flash to obtain a correct exposure. Calculate the shooting distance or aperture value in the following way, and adjust the shooting conditions if the flash output is not sufficient.

ISO Sensitivity	Built-in Flash	ISO Sensitivity	Built-in Flash
	Guide Number		Guide Number
ISO 100	Approx. 12	ISO 3200	Approx. 68
ISO 200	Approx. 17	ISO 6400	Approx. 96
ISO 400	Approx. 24	ISO 12800	Approx. 136
ISO 800	Approx. 34	ISO 25600	Approx. 192
ISO 1600	Approx. 48	ISO 51200	Approx. 272

Calculating Shooting Distance from Aperture Value

The following equation calculates the effective distance of the flash.

Maximum flash distance L1 = Guide number ÷ Aperture value Minimum flash distance L2 = Maximum flash distance ÷ 5 *

*The value 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 F5.6:

L1 = 17 ÷ 5.6 = approx. 3 (m)

L2 = 3 ÷ 5 = approx. 0.6 (m)

Therefore, the flash can be used in a range of about 0.6 m to 3 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 distances closer than 0.7 m, it causes vignetting in the picture corners, light is distributed unevenly and the picture may be overexposed.

Calculating Aperture Value from Shooting Distance

The following equation calculates the correct aperture value. Aperture value F = Guide number ÷ Shooting distance

Example:

When the sensitivity is ISO 200 and the shooting distance is 4 m: F = 17 ÷ 4 = approx. 4.2

If the resulting number (4.2, in the above example) is not available as a lens aperture, a smaller number that is closest (4.0, in the above example) is generally used.