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.

*more...*