1. An optical measuring system for measuring a critical size of a nanostructure, the optical measuring system comprising: an illumination module configured to illuminate a sample with the nanostructure that is an examined object; an optical image generating module configured to generate an optical image of the nanostructure; a defocusing module configured to defocus the optical image; a calculation module configured to calculate a set of images with various defocusing levels; an adjusting and monitoring module configured to adjust and monitor parameters of an optic scheme and illumination conditions with respect to the set of images with various defocusing levels, and transmit, to the calculation module, the parameters of the optic scheme and the illumination conditions; a recording module configured to record the set of images with various defocusing levels; and a comparison module configured to compare the recorded and defocused images with pre-calculated and defocused images.