Features:
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To enable slow shutter speeds to be used to record movement in subjects such as waterfalls, clouds, cars, etc.
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To decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background.
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To decrease the effective ISO of high speed film (ie: above ISO400) and allow it to be used outdoors in bright situations.
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To allow video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes which would normally cause overexposure.