= Solution
The screen now lies in the back <focal plane> of the <lens>. In paraxial <geometrical optics>, a <light ray> arriving at the <lens> at height $y$ and angle $\theta$ leaves at angle $\theta-y/f$, and reaches the screen at
$$
x=y+f(\theta-y/f)=f\theta.
$$
Thus all <light rays> with the same incident direction reach the same screen coordinate, independently of their height at the <lens>. The <Fraunhofer diffraction> pattern is an angular pattern mapped to $x=f\theta$:
$$
\boxed{I(x)=4I_{s,f}\cos^2\left(\frac{\pi sx}{\lambda f}\right),\qquad \Delta x=\frac{\lambda f}{s}.}
$$
Here $I_{s,f}$ is the one-slit <light intensity> after focusing. Its absolute value depends on collected light and the <lens> <aperture>, which are unspecified; it need not equal the no-<lens> value $I_s$. Propagation from the slits to the <lens> introduces a common angular <wave phase>, so it does not alter the relative <wave phase> responsible for these fringes, provided the <aperture> passes the required light.
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