= Solution
In <wave optics>, a finite lens aperture produces <diffraction>. Contributions interfere constructively near the geometric focus but form a finite <point spread function>, rather than a point of infinite intensity. For a uniformly illuminated circular aperture of diameter $D$, the paraxial focal pattern is an <Airy disk>, with first dark-ring radius approximately $1.22\lambda f/D$. Aberrations can broaden or distort it further; even an ideal lens retains the finite diffraction scale.
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