Nanoscale ridge apertures have been demonstrated to be applied for high-resolution lithography. We performed a numerical study of nanoscale bowtie apertures with different outline dimensions and gap sizes to analyze their detailed field distribution for near-field scanning optical lithography (NSOL). It is found that the high image contrast, which is necessary for good quality lithography, is obtained in the near-field region and decays quickly with increasing distance. Furthermore, a smaller gap size achieves higher image contrast and deeper depth of focus. With the NSOL system, static and scanning lithography experiments are conducted. Combined with the passive flexure stage for contact control, we achieved 18-nm lithography resolution.