Question

How the real lens affects light rays that makes it clear that the real lens is less "ideal"? Why are these imperfections a problem when building high-precision optical devices, like telescopes?

Answer 1

1 Aberration is a property of optical systems such as lenses that causes light to be spread out over some region of space rather than focused to a point. Aberrations cause the image formed by a lens to be blurred or distorted, with the nature of the distortion depending on the type of aberration.

2 Diffraction: Diffraction is the slight bending of light as it passes around the edge of an object.

These two properties are make our optical system non ideal.

For the telescopes which which used to observe the astronomical objects or other. Such high precision is required because a small change can affect the output or observations.