Even a perfect lens cannot focus light to a geometric point because of the wave nature of light (Diffraction).

• Light passing through a circular aperture (like a telescope lens) forms a diffraction pattern called the Airy Disk.
• Rayleigh Criterion: Two point sources are just resolved when the central maximum of one pattern falls on the first minimum of the other.
• Smallest resolvable angle: $\Delta \theta =\frac{1.22\lambda }{D}$, where $D$ is the diameter of the aperture.