Pentax K10D User Manual - Page 178

176 Distance and Aperture when Using the Built-in Flash A set criteria is necessary between the guide number, aperture and distance when shooting with the flash. Calculate and adjust the shooting conditions if flash is not sufficient. Built-in flash guide number Sensitivity ISO 100 ISO 200 ISO 400 ISO 800 ISO 1600 Built-in flash guide number 11 15.6 22 31 44 4 Calculating Shooting Distance from Aperture Value Function Reference 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 = 11 ÷ 2.8 = approx. 3.9 (m) L2 = 3.9 ÷ 5 = approx. 0.8 (m) Therefore, the flash can be used in a range of about 0.8 m to 3.9 m. The flash cannot be used when the distance is less than 0.7 m. 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 When sensitivity is [ISO 100] and shooting distance is 5 m, aperture value is: F = 11 ÷ 5 = 2.2 If the resulting number (2.2, in the above example) is not available as a lens aperture, the smaller number that is closest (2, in the above example) is generally used.