f-ratio

• f-ratio is the ratio of focal length to aperture
• Aperture is defined as the focal length/f-ratio.
• Each f-stop increases brightness twice, or EV by 1.
• One f-stop is an increase by a factor of $1/\sqrt{2}$
• A half f-stop is an increase by a factor of $(\frac{1}{\sqrt{2}}{)}^{\frac{1}{2}}$