the defects of lens are following
Light impinging on different areas of the spherical surface of the lens will not meet at precisely the same spot. The rays striking the lens farthest from the center will focus slightly closer to the lens than rays than strike the lens near its center.
Chromatic aberration results from the fact that a lens refracts or bends some colors of light more sharply than others. A lens bends violet light rays more sharply than green, and red suffers even less refraction. As a result, the lens tends to separate white light into its component colors, and a colorful halo results.