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A diaphragm inside the camera lens, known as the aperture, can be opened or nearly closed, to regulate the amount of light that falls on the image sensor. The ratio of the lens’s focal length to the diameter of the opening is the f-number, or f-stop. Due to the way, the values are calculated, the smaller numbers (f1.2, f2.4) actually represent wider openings. Each full f-stop represents a doubling or halving of light. For this reason, photographers often describe shutter speeds in f-stops as well. For example, a shutter speed of 1/60 second is one stop less than 1/30 second.


Also see: Aperture and Shutter Speed.

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Reference: Wikipedia