  Recent studies reports that an epsilon-near-zero (ENZ) thin slab between a
specimen and a substrate contributes in enhancing the spatial resolution of the
optical system. Here, we investigate the ENZ thickness dependence of the
resolution enhancement. By employing the edge response function, the resolution
of the optical system is directly measured when imaging a sharp edge of a metal
film. We found that the optimum ENZ slab thickness was 700 nm and the achieved
resolution was 11 {\mu}m at the wavelength of 8 {\mu}m. Owing to the enhanced
resolution by ENZ slab, we successfully imaged the subwavelength slit arrays.
