Astigmatism (for a human eye) can be removed by using

The lens used to correct long-sightedness (hypermetropia) is:

$A$ person can see clearly only up to a distance of $25 \, cm$. He wants to read a book placed at a distance of $50 \, cm$. What kind of lens does he require for his spectacles and what must be its power?

The image formed on the retina is:

$A$ person can see objects clearly only when they lie between $50\, cm$ and $400\, cm$ from his eyes. In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens the person has to use will be

For the myopic eye,the defect is cured by