The recently predicted phenomenon of diffraction lensless electromagnetic focusing in small volume of the near zone of a sub-wavelength nanoaperture (Semiconductors, 2020, Vol. 54, p. 1814) is analyzed theoretically. For this, we use the rigorous solution of the problem of plane wave diffraction by a slot in a perfectly conducting screen of arbitrary thickness with a thin dielectric film on a substrate, which plays a part of a radiation receiver (detector) behind the slot. The scalar focusing parameter is introduced for estimation of quality of a diffraction image of a slot in a film from the standpoint of local spectroscopy and optical lithography. It is convenient to use for search of domains of variability of the problem parameters, where the phenomenon of sub-wavelength focusing exists.