Using the finite-difference time-domain simulation, we show that if a gradient-index or binary planar dielectric microlens that focuses light at the output surface has a near-focus subwavelength slit the focal spot width is determined by the slit width. Notably, the slit allows the output light proportion to be increased due to the surface wave scattering, thus forming a focal spot nearly devoid of side lobes. In this work, the focal spot width of λ/23 and the diffraction efficiency of focusing of 44% are achieved.