F-stop
From The Glossary of Digital Photography
A number calculated by dividing the focal length of the lens by the diameter of the diaphragm opening. The term can be written as f-stop or f-stop. The difference between f-stop and f-number is that f-number is a continuous number and f-stop is adjusted to certain discrete steps. The standard, full f-stops are f/1.0, f/1.4, f /2.0, f/2.8, f/4, f/5.6, f/8, f /11, f/16, f/22, f/32, f/45, f/64, and so on. Notice how the numbers increase by multiplying by the square root of 2, which is approximately 1.4, while each step also represents one-half the amount of light. The numbers are rounded off to make them easier to deal with, especially when mentally calculating exposures. Since an f-stop is a ratio, it should be notated as f/1.4, but often the slash (/) is left out. Many cameras allow for half stops or third stops. See also fractional stops
