Contact lithography using bow-tie-shaped nano-apertures was recently demonstrated to achieve nanometer scale resolution. In this work, the detailed field distributions in contact nanolithography are analyzed using finite difference time domain simulations. It was found that the high imaging contrast, which is necessary for successful lithography, is achieved close to the mask exit plane and decays quickly with the increase of the distance from the mask exit plane. Simulations are also performed for comparable regular-shaped apertures and different shape bow-tie apertures. Design rules are proposed to optimize the bow-tie aperture for producing a sub-wavelength, high transmission field with high imaging contrast.