104Distance and Aperture when Using the Built-in Flash■ A set criteria is necessary between the guide number, aperture and distancewhen shooting with the flash. Calculate and adjust the shooting conditions ifflash is not sufficient.● Built-in flash guide numberSensitivity Built-in flash guide numberISO 100 13ISO 200 18.4ISO 400 26ISO 800 36.7ISO 1600 52Calculating Aperture Value from Shooting DistanceThe following equation calculates the aperture value for shooting distances.Aperture Value Used F = Guide number ÷ Shooting distanceExample)When sensitivity is [ISO 200] and shooting distance is 6.1 m, aperture value is:F = 18.4 ÷ 6.1 = 3If the resulting number (3, in the above example) is not available as a lens aperture,the smaller number that is closest (2.8, in the above example) is generally used.Using the Built-in FlashCalculating Shooting Distance from Aperture Value■ The following equation calculates the distance of the flash for aperturevalues.Maximum flash distance L1 = Guide number ÷ Selected apertureMinimum flash distance L2 = Maximum flash distance L1 ÷ 5** The value 5 used in the formula above isa fixed value which applies only whenusing the built-in flash alone.Example)When sensitivity is [ISO 200] and aperture value is F4L1 = 18.4 ÷ 4 = 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 cannotbe used when the distance is less than 0.8 m. When the flash is used at closer than0.8 m, it causes vignetting in the picture corners, light is distributed unevenly and thepicture may be over-exposed.
