Pentax DS Operation Manual - Page 151

149 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. Flash guide number for built-in flash Sensitivity Built-in flash guide number ISO200 15.6 ISO400 22 ISO800 31 ISO1600 44 ISO3200 62 5 Function Reference 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 [ISO200] and aperture value is F4 L1 = 15.6 ÷ 4 = 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 overexposed.