The distance of the eye-lens from the retina is x. For a normal eye, the maximum focal length of the eye-lens

$\text{ For normal eye focal length is maximum when eye is least strained i.e., when object is at }$$\text{ infinity and image formed is at retina (focus of lens) }$

$\therefore \text{ focal length }=x\text{ for maximum focal length }$

$$\begin{gathered} \text{ By lens formula } \\ \frac{1}{v}-\frac{1}{u}=\frac{1}{f} \\ \Rightarrow \frac{1}{x}-\frac{1}{(-\infty )}=\frac{1}{f} \\ \Rightarrow f=x \end{gathered}$$