AUTOmatic or computer flash units are designed to give correct amount of flash illumination for subject of various distances without the need for calculations based on a guide no............They do this by adjusting the duration of the flash automatically according to the subject range.
BEcause an auto flash guages the exposure according to the amount of light reflected from the subject, it also automatically takes into account the overall lightness or darkness of the scene in front of it. When the light sensor reads that the subject has received the enough light for the ISO set and aperture in use, it cut off the light.
SO all thing being equal, the closure the subject is to the camera, the shorter the total flash duration. u just set an aperture to suit your ISO speed, and keep that aperture for all distances within the flash range and your subject will be correctly exposed. ........what the newbies do. they set the aperture at f5.6 and flash on auto at f.5.6 at ISO 100, they go on exposing and slightly increase the aperture to f4, f3.5 in long distances (12 to 15 ft above) and get the correct exposure.......nowdays the ultimate weapon is the auto flash gun for photographers.
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