In photography, the f-number is defined as the ratio of the lens’s focal length to the diameter of its aperture.
In a DSLR camera, the F-number could easily been seen usually on the first line of the back screen (as showed in the picture above). As you can see, the measuring scale of F-number is 1 () , 1.4 (), 2 (), 2.8 (), etc upto . From appearance, this scale should have be invented by an engineer.
For a fixed lens, the focal length could not be changed, so the F-number is also used to estimate the lens’ aperture. The higher the F-number, the smaller the aperture.
It turns out that the F-number, defined this way, is proportional to the depth of field. It is easier to remember that to increase the depth of field, you increase the F-number.
Here is a picture with F-number equal to 25, because some monuments of the city (Paris) in the background was supposed to visible.