<Dd> BFD = f (1 − (n − 1) d n R 1). (\ displaystyle (\ mbox (BFD)) = f \ left (1 - (\ frac ((n - 1) d) (nR_ (1))) \ right).) </Dd> <P> In the sign convention used here, the value of R will be positive if the first lens surface is convex, and negative if it is concave . The value of R is negative if the second surface is convex, and positive if concave . Note that sign conventions vary between different authors, which results in different forms of these equations depending on the convention used . </P> <P> For a spherically curved mirror in air, the magnitude of the focal length is equal to the radius of curvature of the mirror divided by two . The focal length is positive for a concave mirror, and negative for a convex mirror . In the sign convention used in optical design, a concave mirror has negative radius of curvature, so </P> <Dl> <Dd> f = − R 2, (\ displaystyle f = - (R \ over 2),) </Dd> </Dl>

What is a focal length of a lens