My question is about ray optics particularly image defects

When a ray crosses a boundary between materials with different refractive index, at an angle, it bends.

Here is the ray going straight without bending.

But if it's at an angle, then it does bend.

At a bigger angle it bends more.

If it's a spherical lens, then one or both surfaces fits the shape of part of a sphere.

So the farther you get from the center, the bigger the angle the light ray meets..

So those rays bend more.

Here are two ray diagrams which show the refraction of light by a plano-convex lens.

The right hand diagram is for an ideal lens where all incident rays parallel to the principal axis after refraction pass through the focal point, $F$, on the principal axis.

The left hand diagram is for a lens with the right-hand surface being part of a sphere.
Rays close to the principal axis meet in a small region which is the focal "point" but the incident parallel rays a refracted "too much" and miss the focal "point".

The effect is similar when one considers a bi-convex lens and spherical mirrors.