The phenomenon of spatial frequency doubling generated by a two-step technique with the collimated beam at normal incidence in the second exposure is presented. Theoretical analysis demonstrates that the phenomenon is induced by the Talbot effect in photoresist and the superposition of two exposures, and the minimum achievable period of the grating with double spatial frequency can be close to one half of the exposure wavelength in vacuum, divided by the refractive index of photoresist λ/2n. The two-step technique has the potential to be a simpler and more practical resolution-improving technique for the Talbot-effect-based approach of spatial frequency doubling.