The optical image of a structured target, where a particular structure repeats itself several times, varies greatly as it is moved through focus if the spacing between the structures is such that the scattered field from the edges interferes. This condition results in a different and complex optical response compared to that found for structures much farther apart. The complex optical image of a structured target in the proximity region is sensitive to the dimensions of the target and the optical parameters. By appropriately analyzing the through-focus optical image, information can be obtained regarding the target and the optical system. In the present work an array of lines is used as a structured target. Experimental data were obtained using a bright field microscope, and results were simulated using a 'modal diffraction grating model' (also known as a rigorous coupled-wave analysis (RCWA)). The gradient-energy focus-metric method was used to characterize the through-focus optical response. The resultant focus metric signature is sensitive to changes in the line width in the nanometer range, giving it potential for metrology applications and characterization of optical tools.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.