Angela takes a photo of her friend's track meet with a shutter speed of 1/250 second at f/8. She likes the exposure of this picture, but her friend, running a lap, is blurred. Angela resets her camera with a shutter speed one fourth as long to get a better picture on the next lap. What f-stop should she use on her next picture?
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Since the photo is blurred, too much light is getting into the camera. By setting the shutter speed to 1/4 of its current setting, the camera will be exposed for a much shorter period of time, but this will reduce the exposure, which Angela wants to remain constant. Therefore the aperture should increase (a decrease in f-stop value) in order to let more light in that way, compensating for the decrease in shutter speed.
The general rule is an inverse relationship; for every stop taken from one of these settings, give one stop to the other. You basically need to either memorize the stop values, or have a chart of them like the one below.
Since 1/4 of 1/250 is 1/1000, this is a change of two stops (from 1/250 to 1/500 to 1/1000) so our aperture should open up by two stops, to f/4.
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