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Behind the lens of your digital camera is a diaphragm that opens and closes to determine the amount of light falling on to the CCD/CMOS sensor or film. By altering the aperture, you can also change the depth of field.
Common f-stops are as follows (all spaced by a full stop):
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, etc