In a previous article we looked at how lenses work. We learned about the focal length. Today we’ll focus on camera lenses and what the term f-stop means.
The term f-stop is a ratio. It has no dimensions. You don’t measure an f-stop in meters, inches, kilograms or even degrees Fahrenheit. An f-stop is the ratio of two distances. It’s the ratio of the focal length of a lens to its diameter. In figure 1, the f-stop is f/d _where _f is the focal length and _d _is the diameter.