Provided is an optical measurement system. The optical measurement system includes: an optical module which includes an optical system and which is configured to illuminate a sample and register a defocused image of a nanostructured surface of the sample, an optical system parameter control module configured to set optical parameters of the optical system, an optical transfer function (OTF) measurement module configured to measure an OTF, a defocused image calculation module configured to calculate the defocused image based on the measured OTF and the optical parameters, and a critical dimension (CD) evaluation module configured to compare the registered defocused image with the calculated defocused image of the nanostructured surface of the sample and to output a CD value of the nanostructured surface.