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Fill lighting can be required indoors or out. Above you see an example of a photo that needs help from the fill FLASH. The photo on the left was made with the light meter in the camera just set at normal. The bright light in the background fooled the meter into thinking there was more light then there was. The exposure was made at f16 at 1/60 using Tri-x. But, the result was not good for our subject. An actual light meter reading of JUST the background reveals an exposure of f22. Tilting the camera down or moving in close to the subject so no background is showing reveled a subject brightness of f2. This is perfect for a fill flash.
To do this we must FIRST set the light meter to the SYNC speed, usually 60, on our camera. Then we take a light meter reading of the brightest area of the scene. In this case it was f22. Now we look at the flash exposure table on the back of most strobes. They will usually have an exposure based on the following formula. f stop = GN / D where GN is a Guide Number given to us by the strobe manufacturer. In this case that number is 100. In this case we know what the f-stop must be so we have a new formula of 22=100 / D. Now we can find D by dividing the 22 into the 100 to get 4.5 feet. If we wanted the fill to be just a little less bright than the background we would use f 16 which would then be 100 / 16 or 6.25 feet. To take the photograph on the right we then put on the strobe, set the shutter to the sync speed of 60 and the aperture to f22 and stand between 4 and a half to 6 and a half feet from our subject.
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