Far-field super-resolution imaging techniques such as stimulated emission depletion (STED), stochastic optical reconstruction microscopy (STORM), and photoactivation localization microscopy (PALM) etc., have enabled fluorescence labeled nanoscale imaging and analysis. However, it is necessary or preferable to develop label-free high resolution imaging techniques to avoid the problems of phototoxicity with live cells and moreover, for many non-biological applications where fluorescent labelling is simply not feasible. Recently, by capturing the exponentially decaying evanescent waves and propagating it to far field, novel imaging technique like hyperlens and microsphere contacting technologies, micro-fiber and nanowire illumination technology have brought new opportunities to label-free super resolution imaging, but still suffer from weak signal compared to background noise. On the other hand, there is lack of criteria reported to quantify the imaging quality of label free far-field nanoscopy, that will slow down the developing of label-free far-field nanoscopy. We introduce CNR first to label-free far-field nanoscopy to quantify the imaging performance and investigate the key influencing elements systematically. In our study, we investigated and analyzed the key elements to achieve high CNR label-free wide-field far-field sub-diffraction imaging. By optimizing the key factors including polarization, materials, as well as fabrication conditions, sub-diffraction imaging with resolution of 122 nm in a large FOV has been achieved experimentally. This work has provided an efficient and convenient way to realize high contrast wide-field far-field label-free super-resolution based on evanescent wave illumination.