Pentax K20D Operation Manual - Page 143

Distance and Aperture when Using 141 the Built-in Flash Using the 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 ISO 100 ISO 200 ISO 400 ISO 800 ISO 1600 ISO 3200 Built-in flash guide number 13 18.4 26 36.8 52 73.5 Calculating Shooting Distance from Aperture Value 5 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.