Chromatic aberration

Equation 6.3 shows that $f$ varies as $n$ varies. Generally, $n$ is larger for shorter $\lambda$, and it follows that $f$ is shorter when $\lambda$ is smaller. Lenses have a shorter focal length for blue light than for red. The change in $f$ can be found by differentiating equation 6.3 -

$$(\delta f)/f = - (\delta n)/(n-1)$$ (64)